feat(storage): serve media through backend and add local FS backend

- introduce storage.Backend with S3 and local file-system implementations
- deliver all blobs via signed GET/HEAD /api/objects/<key> (Range, Content-Type)
- stream S3 objects through the backend; remove presigned URL path
- share shared HMAC Signer between backends
- remove S3_PUBLIC_URL; rename S3_PRESIGN_TTL to OBJECT_URL_TTL
- make storage label in health dynamic; nginx proxy_buffering off for objects

backend/internal/albums/service.go

@@ -21,10 +21,10 @@ type Service struct {
 	store   *Store
 	members MemberLookup
 	hub     *sse.Hub
-	storage *storage.Client
+	storage storage.Backend
 }
 
-func NewService(store *Store, members MemberLookup, hub *sse.Hub, storage *storage.Client) *Service {
+func NewService(store *Store, members MemberLookup, hub *sse.Hub, storage storage.Backend) *Service {
 	return &Service{store: store, members: members, hub: hub, storage: storage}
 }
 
@@ -61,12 +61,12 @@ func (s *Service) List(ctx context.Context, userID string) ([]models.Album, erro
 	return out, nil
 }
 
-// attachCover fills CoverThumbURL from the presigned thumbnail key, when present.
+// attachCover fills CoverThumbURL from the signed thumbnail URL, when present.
 func (s *Service) attachCover(ctx context.Context, a *models.Album) {
 	if s.storage == nil || a == nil || a.CoverThumbKey == "" {
 		return
 	}
-	if url, err := s.storage.PresignGet(ctx, a.CoverThumbKey); err == nil {
+	if url, err := s.storage.URL(ctx, a.CoverThumbKey); err == nil {
 		a.CoverThumbURL = url
 	}
 }

backend/internal/app/app.go

@@ -2,6 +2,7 @@ package app
 
 import (
 	"context"
+	"errors"
 	"io/fs"
 	"net/http"
 	"strings"
@@ -27,7 +28,7 @@ import (
 type Server struct {
 	cfg     *config.Config
 	pool    *pgxpool.Pool
-	storage *storage.Client
+	storage storage.Backend
 	hub     *sse.Hub
 
 	albumsStore *albums.Store
@@ -47,7 +48,7 @@ type Server struct {
 	share *sharelinks.Service
 }
 
-func New(cfg *config.Config, pool *pgxpool.Pool, st *storage.Client, hub *sse.Hub) *Server {
+func New(cfg *config.Config, pool *pgxpool.Pool, st storage.Backend, hub *sse.Hub) *Server {
 	authStore := auth.NewStore(pool)
 	tm := auth.NewTokenManager(cfg.Auth.JWTSecret, cfg.Auth.AccessTTL, cfg.Auth.RefreshTTL)
 	authSvc := auth.NewService(authStore, tm)
@@ -109,6 +110,8 @@ func (s *Server) Router() *gin.Engine {
 
 	api := r.Group("/api")
 	api.GET("/health", s.health)
+	api.GET("/objects/*key", s.serveObject)
+	api.HEAD("/objects/*key", s.serveObject)
 
 	api.POST("/auth/register", authHandler.Register)
 	api.POST("/auth/login", authHandler.Login)
@@ -183,17 +186,37 @@ func (s *Server) health(c *gin.Context) {
 	if err := s.pool.Ping(ctx); err != nil {
 		dbStatus = "error"
 	}
-	s3Status := "ok"
+	storageLabel := s.cfg.Storage.Backend
+	if storageLabel == "" {
+		storageLabel = "s3"
+	}
+	storeStatus := "ok"
 	if err := s.storage.Ping(ctx); err != nil {
-		s3Status = "error"
+		storeStatus = "error"
 	}
 	status := http.StatusOK
 	overall := "ok"
-	if dbStatus != "ok" || s3Status != "ok" {
+	if dbStatus != "ok" || storeStatus != "ok" {
 		status = http.StatusServiceUnavailable
 		overall = "degraded"
 	}
-	c.JSON(status, gin.H{"status": overall, "db": dbStatus, "s3": s3Status})
+	c.JSON(status, gin.H{"status": overall, "db": dbStatus, storageLabel: storeStatus})
+}
+
+func (s *Server) serveObject(c *gin.Context) {
+	key := strings.TrimPrefix(c.Param("key"), "/")
+	switch err := s.storage.ServeObject(c.Writer, c.Request, key); {
+	case err == nil:
+		return
+	case errors.Is(err, storage.ErrSignedURLInvalid):
+		c.Status(http.StatusForbidden)
+	case errors.Is(err, storage.ErrObjectNotFound):
+		httpx.RespondError(c, httpx.NotFound("object_not_found", "object not found"))
+	case errors.Is(err, storage.ErrRangeNotSatisfiable):
+		c.Status(http.StatusRequestedRangeNotSatisfiable)
+	default:
+		c.Status(http.StatusInternalServerError)
+	}
 }
 
 func (s *Server) albumSSE(c *gin.Context) {

backend/internal/app/health_test.go

@@ -0,0 +1,71 @@
+package app
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"shared-album/internal/config"
+	"shared-album/internal/storage"
+)
+
+func mustSigner(t *testing.T) *storage.Signer {
+	t.Helper()
+	signer, err := storage.NewSigner("http://app.test", "secret", time.Hour)
+	if err != nil {
+		t.Fatalf("signer: %v", err)
+	}
+	return signer
+}
+
+func lazyPool(t *testing.T) *pgxpool.Pool {
+	t.Helper()
+	pool, err := pgxpool.New(context.Background(), "postgres://u:p@127.0.0.1:1/x?sslmode=disable")
+	if err != nil {
+		t.Fatalf("pool: %v", err)
+	}
+	t.Cleanup(pool.Close)
+	return pool
+}
+
+func TestHealthStorageLabel(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	fs, err := storage.NewFS(t.TempDir(), mustSigner(t))
+	if err != nil {
+		t.Fatalf("fs: %v", err)
+	}
+	srv := &Server{cfg: &config.Config{}, pool: lazyPool(t), storage: fs}
+
+	call := func() map[string]any {
+		rec := httptest.NewRecorder()
+		c, _ := gin.CreateTestContext(rec)
+		c.Request = httptest.NewRequest(http.MethodGet, "/api/health", nil)
+		srv.health(c)
+		var body map[string]any
+		if err := json.Unmarshal(rec.Body.Bytes(), &body); err != nil {
+			t.Fatalf("decode: %v", err)
+		}
+		return body
+	}
+
+	srv.cfg.Storage.Backend = "fs"
+	if body := call(); body["fs"] != "ok" {
+		t.Fatalf("expected fs label ok, got %v", body)
+	}
+
+	srv.cfg.Storage.Backend = "s3"
+	if body := call(); body["s3"] != "ok" {
+		t.Fatalf("expected s3 label ok, got %v", body)
+	}
+
+	srv.cfg.Storage.Backend = ""
+	if body := call(); body["s3"] != "ok" {
+		t.Fatalf("expected default s3 label, got %v", body)
+	}
+}

backend/internal/app/objects_test.go

@@ -0,0 +1,110 @@
+package app
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/gin-gonic/gin"
+
+	"shared-album/internal/config"
+	"shared-album/internal/sse"
+	"shared-album/internal/storage"
+)
+
+func newTestServerWithFS(t *testing.T) (*storage.FS, *gin.Engine) {
+	t.Helper()
+	gin.SetMode(gin.TestMode)
+	signer, err := storage.NewSigner("http://app.test", "secret", time.Hour)
+	if err != nil {
+		t.Fatalf("signer: %v", err)
+	}
+	fs, err := storage.NewFS(t.TempDir(), signer)
+	if err != nil {
+		t.Fatalf("new fs: %v", err)
+	}
+	srv := New(&config.Config{}, nil, fs, sse.NewHub(1))
+	return fs, srv.Router()
+}
+
+func TestServeObjectRoute(t *testing.T) {
+	ctx := context.Background()
+	fs, r := newTestServerWithFS(t)
+	key := "objects/abc/original.jpg"
+	if err := fs.Put(ctx, key, strings.NewReader("0123456789"), 10, "image/jpeg"); err != nil {
+		t.Fatalf("put: %v", err)
+	}
+	raw, err := fs.URL(ctx, key)
+	if err != nil {
+		t.Fatalf("url: %v", err)
+	}
+	u, err := url.Parse(raw)
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+
+	do := func(target string, rangeHeader string) *httptest.ResponseRecorder {
+		req := httptest.NewRequest(http.MethodGet, target, nil)
+		if rangeHeader != "" {
+			req.Header.Set("Range", rangeHeader)
+		}
+		rec := httptest.NewRecorder()
+		r.ServeHTTP(rec, req)
+		return rec
+	}
+
+	t.Run("valid", func(t *testing.T) {
+		rec := do(u.Path+"?"+u.RawQuery, "")
+		if rec.Code != http.StatusOK {
+			t.Fatalf("status = %d, want 200", rec.Code)
+		}
+		if rec.Body.String() != "0123456789" {
+			t.Fatalf("body = %q", rec.Body.String())
+		}
+	})
+
+	t.Run("range", func(t *testing.T) {
+		rec := do(u.Path+"?"+u.RawQuery, "bytes=2-5")
+		if rec.Code != http.StatusPartialContent {
+			t.Fatalf("status = %d, want 206", rec.Code)
+		}
+		if rec.Body.String() != "2345" {
+			t.Fatalf("body = %q, want 2345", rec.Body.String())
+		}
+	})
+
+	t.Run("expired", func(t *testing.T) {
+		q := u.Query()
+		q.Set("e", strconv.FormatInt(time.Now().Add(-time.Minute).Unix(), 10))
+		rec := do(u.Path+"?"+q.Encode(), "")
+		if rec.Code != http.StatusForbidden {
+			t.Fatalf("status = %d, want 403", rec.Code)
+		}
+	})
+
+	t.Run("forged", func(t *testing.T) {
+		q := u.Query()
+		q.Set("s", "deadbeef")
+		rec := do(u.Path+"?"+q.Encode(), "")
+		if rec.Code != http.StatusForbidden {
+			t.Fatalf("status = %d, want 403", rec.Code)
+		}
+	})
+
+	t.Run("missing", func(t *testing.T) {
+		raw, err := fs.URL(ctx, "objects/def/original.jpg")
+		if err != nil {
+			t.Fatalf("url: %v", err)
+		}
+		mu, _ := url.Parse(raw)
+		rec := do(mu.Path+"?"+mu.RawQuery, "")
+		if rec.Code != http.StatusNotFound {
+			t.Fatalf("status = %d, want 404", rec.Code)
+		}
+	})
+}

backend/internal/app/share_flow_test.go

@@ -0,0 +1,211 @@
+package app
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"image"
+	"image/color"
+	"image/png"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"os"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"shared-album/internal/albums"
+	"shared-album/internal/auth"
+	"shared-album/internal/config"
+	"shared-album/internal/members"
+	"shared-album/internal/models"
+	"shared-album/internal/photos"
+	"shared-album/internal/sharelinks"
+	"shared-album/internal/sse"
+	"shared-album/internal/storage"
+)
+
+func integrationPool(t *testing.T) *pgxpool.Pool {
+	t.Helper()
+	dsn := os.Getenv("TEST_DATABASE_URL")
+	if dsn == "" {
+		t.Skip("TEST_DATABASE_URL not set")
+	}
+	pool, err := pgxpool.New(context.Background(), dsn)
+	if err != nil {
+		t.Skipf("cannot connect: %v", err)
+	}
+	if err := pool.Ping(context.Background()); err != nil {
+		t.Skipf("cannot ping: %v", err)
+	}
+	t.Cleanup(pool.Close)
+	return pool
+}
+
+func makeTestPNG(t *testing.T) []byte {
+	t.Helper()
+	img := image.NewRGBA(image.Rect(0, 0, 4, 4))
+	for x := 0; x < 4; x++ {
+		for y := 0; y < 4; y++ {
+			img.Set(x, y, color.RGBA{R: uint8(x * 40), G: uint8(y * 40), B: 128, A: 255})
+		}
+	}
+	var buf bytes.Buffer
+	if err := png.Encode(&buf, img); err != nil {
+		t.Fatalf("encode png: %v", err)
+	}
+	return buf.Bytes()
+}
+
+// runShareFlow uploads a photo and validates the read-only share flow returns a
+// usable photo URL. verify inspects/serves the URL according to the backend.
+func runShareFlow(t *testing.T, backend storage.Backend, verify func(t *testing.T, router *gin.Engine, raw string)) {
+	t.Helper()
+	ctx := context.Background()
+	pool := integrationPool(t)
+	gin.SetMode(gin.TestMode)
+
+	hub := sse.NewHub(1)
+	srv := New(&config.Config{}, pool, backend, hub)
+	router := srv.Router()
+
+	authStore := auth.NewStore(pool)
+	owner := &models.User{ID: auth.NewID(), Email: auth.NewID()[:8] + "@test.local", PasswordHash: "x", Name: "Owner"}
+	if err := authStore.CreateUser(ctx, owner); err != nil {
+		t.Fatalf("create user: %v", err)
+	}
+
+	albumsStore := albums.NewStore(pool)
+	albumsSvc := albums.NewService(albumsStore, members.NewStore(pool), hub, backend)
+	album, err := albumsSvc.Create(ctx, owner.ID, albums.CreateInput{Title: "Share Album"})
+	if err != nil {
+		t.Fatalf("create album: %v", err)
+	}
+
+	photosSvc := photos.NewService(photos.NewStore(pool), albumsSvc, backend, hub)
+	pngData := makeTestPNG(t)
+	if _, err := photosSvc.Upload(ctx, photos.UploadInput{
+		AlbumID: album.ID, UserID: owner.ID, Filename: "pic.png", Data: pngData,
+	}); err != nil {
+		t.Fatalf("upload: %v", err)
+	}
+
+	var keys []string
+	rows, err := pool.Query(ctx,
+		`SELECT o.storage_key FROM photo_objects po
+		 JOIN objects o ON o.id = po.object_id
+		 JOIN photos p ON p.id = po.photo_id
+		 WHERE p.album_id = $1`, album.ID)
+	if err == nil {
+		for rows.Next() {
+			var k string
+			if rows.Scan(&k) == nil {
+				keys = append(keys, k)
+			}
+		}
+		rows.Close()
+	}
+	t.Cleanup(func() {
+		_, _ = pool.Exec(ctx, `DELETE FROM photos WHERE album_id = $1`, album.ID)
+		for _, k := range keys {
+			_, _ = pool.Exec(ctx, `DELETE FROM objects WHERE storage_key = $1`, k)
+		}
+		_ = backend.Delete(context.Background(), keys...)
+		_, _ = pool.Exec(ctx, `DELETE FROM albums WHERE id = $1`, album.ID)
+		_, _ = pool.Exec(ctx, `DELETE FROM users WHERE id = $1`, owner.ID)
+	})
+
+	shareSvc := sharelinks.NewService(sharelinks.NewStore(pool), albumsSvc)
+	link, err := shareSvc.Create(ctx, album.ID, owner.ID, time.Hour)
+	if err != nil {
+		t.Fatalf("share create: %v", err)
+	}
+
+	rec := httptest.NewRecorder()
+	router.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/api/share/"+link.Token+"/photos", nil))
+	if rec.Code != http.StatusOK {
+		t.Fatalf("share photos status = %d body=%s", rec.Code, rec.Body.String())
+	}
+
+	var body struct {
+		Photos []models.Photo `json:"photos"`
+	}
+	if err := json.Unmarshal(rec.Body.Bytes(), &body); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(body.Photos) != 1 {
+		t.Fatalf("photos len = %d, want 1", len(body.Photos))
+	}
+	if body.Photos[0].OriginalURL == "" {
+		t.Fatalf("empty original url")
+	}
+	verify(t, router, body.Photos[0].OriginalURL)
+}
+
+func TestShareFlowWithFS(t *testing.T) {
+	signer, err := storage.NewSigner("http://app.test", "secret", time.Hour)
+	if err != nil {
+		t.Fatalf("signer: %v", err)
+	}
+	fs, err := storage.NewFS(t.TempDir(), signer)
+	if err != nil {
+		t.Fatalf("new fs: %v", err)
+	}
+	runShareFlow(t, fs, func(t *testing.T, router *gin.Engine, raw string) {
+		if !strings.Contains(raw, "/api/objects/") {
+			t.Fatalf("expected fs signed url, got %q", raw)
+		}
+		u, err := url.Parse(raw)
+		if err != nil {
+			t.Fatalf("parse url: %v", err)
+		}
+		rec := httptest.NewRecorder()
+		router.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, u.Path+"?"+u.RawQuery, nil))
+		if rec.Code != http.StatusOK {
+			t.Fatalf("fs object fetch status = %d", rec.Code)
+		}
+	})
+}
+
+func TestShareFlowWithS3(t *testing.T) {
+	endpoint := os.Getenv("S3_ENDPOINT")
+	if endpoint == "" {
+		t.Skip("S3_ENDPOINT not set")
+	}
+	signer, err := storage.NewSigner("http://app.test", "secret", time.Hour)
+	if err != nil {
+		t.Skipf("signer init: %v", err)
+	}
+	cl, err := storage.NewS3(context.Background(), config.S3{
+		Endpoint:     endpoint,
+		Region:       os.Getenv("S3_REGION"),
+		AccessKey:    os.Getenv("S3_ACCESS_KEY"),
+		SecretKey:    os.Getenv("S3_SECRET_KEY"),
+		Bucket:       os.Getenv("S3_BUCKET"),
+		UsePathStyle: true,
+	}, signer)
+	if err != nil {
+		t.Skipf("storage init: %v", err)
+	}
+	runShareFlow(t, cl, func(t *testing.T, router *gin.Engine, raw string) {
+		if !strings.Contains(raw, "/api/objects/") {
+			t.Fatalf("expected backend url, got %q", raw)
+		}
+		if strings.Contains(raw, "X-Amz-Signature") {
+			t.Fatalf("presigned url leaked to client: %q", raw)
+		}
+		u, err := url.Parse(raw)
+		if err != nil {
+			t.Fatalf("parse url: %v", err)
+		}
+		rec := httptest.NewRecorder()
+		router.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, u.Path+"?"+u.RawQuery, nil))
+		if rec.Code != http.StatusOK {
+			t.Fatalf("s3 object fetch status = %d", rec.Code)
+		}
+	})
+}

backend/internal/config/config.go

@@ -28,8 +28,6 @@ type S3 struct {
 	SecretKey    string
 	Bucket       string
 	UsePathStyle bool
-	PublicURL    string
-	PresignTTL   time.Duration
 }
 
 type Auth struct {
@@ -54,9 +52,17 @@ type Worker struct {
 	PendingTTL time.Duration
 }
 
+type Storage struct {
+	Backend   string
+	FSRoot    string
+	URLSecret string
+	URLTTL    time.Duration
+}
+
 type Config struct {
 	Postgres Postgres
 	S3       S3
+	Storage  Storage
 	Auth     Auth
 	Server   Server
 	SSE      SSE
@@ -79,9 +85,9 @@ func Load() (*Config, error) {
 		return nil, fmt.Errorf("invalid JWT_REFRESH_TTL: %w", err)
 	}
 
-	presignTTL, err := time.ParseDuration(getEnv("S3_PRESIGN_TTL", "1h"))
+	urlTTL, err := time.ParseDuration(getEnv("OBJECT_URL_TTL", "1h"))
 	if err != nil {
-		return nil, fmt.Errorf("invalid S3_PRESIGN_TTL: %w", err)
+		return nil, fmt.Errorf("invalid OBJECT_URL_TTL: %w", err)
 	}
 
 	heartbeat, err := time.ParseDuration(getEnv("SSE_HEARTBEAT_INTERVAL", "25s"))
@@ -130,8 +136,12 @@ func Load() (*Config, error) {
 			SecretKey:    getEnv("S3_SECRET_KEY", ""),
 			Bucket:       getEnv("S3_BUCKET", "albums"),
 			UsePathStyle: pathStyle,
-			PublicURL:    getEnv("S3_PUBLIC_URL", ""),
-			PresignTTL:   presignTTL,
+		},
+		Storage: Storage{
+			Backend:   getEnv("STORAGE_BACKEND", "s3"),
+			FSRoot:    getEnv("FS_ROOT", ""),
+			URLSecret: getEnv("URL_SECRET", ""),
+			URLTTL:    urlTTL,
 		},
 		Auth: Auth{
 			JWTSecret:  getEnv("JWT_SECRET", ""),
@@ -160,6 +170,16 @@ func Load() (*Config, error) {
 		return nil, fmt.Errorf("PG_PASSWORD is required")
 	}
 
+	switch cfg.Storage.Backend {
+	case "s3":
+	case "fs":
+		if cfg.Storage.FSRoot == "" {
+			return nil, fmt.Errorf("FS_ROOT is required when STORAGE_BACKEND=fs")
+		}
+	default:
+		return nil, fmt.Errorf("invalid STORAGE_BACKEND %q (want s3 or fs)", cfg.Storage.Backend)
+	}
+
 	return cfg, nil
 }
 

backend/internal/config/config_test.go

@@ -0,0 +1,70 @@
+package config
+
+import (
+	"testing"
+	"time"
+)
+
+func setRequiredEnv(t *testing.T) {
+	t.Helper()
+	t.Setenv("JWT_SECRET", "test-secret")
+	t.Setenv("PG_PASSWORD", "test-pass")
+}
+
+func TestLoadStorageBackend(t *testing.T) {
+	t.Run("defaults to s3", func(t *testing.T) {
+		setRequiredEnv(t)
+		t.Setenv("STORAGE_BACKEND", "")
+		cfg, err := Load()
+		if err != nil {
+			t.Fatalf("load: %v", err)
+		}
+		if cfg.Storage.Backend != "s3" {
+			t.Fatalf("backend = %q, want s3", cfg.Storage.Backend)
+		}
+	})
+
+	t.Run("fs with root", func(t *testing.T) {
+		setRequiredEnv(t)
+		t.Setenv("STORAGE_BACKEND", "fs")
+		t.Setenv("FS_ROOT", "/var/lib/galo/objects")
+		cfg, err := Load()
+		if err != nil {
+			t.Fatalf("load: %v", err)
+		}
+		if cfg.Storage.Backend != "fs" || cfg.Storage.FSRoot != "/var/lib/galo/objects" {
+			t.Fatalf("storage = %+v", cfg.Storage)
+		}
+	})
+
+	t.Run("fs without root", func(t *testing.T) {
+		setRequiredEnv(t)
+		t.Setenv("STORAGE_BACKEND", "fs")
+		t.Setenv("FS_ROOT", "")
+		t.Setenv("URL_SECRET", "")
+		if _, err := Load(); err == nil {
+			t.Fatal("expected error for fs without FS_ROOT")
+		}
+	})
+
+	t.Run("invalid backend", func(t *testing.T) {
+		setRequiredEnv(t)
+		t.Setenv("STORAGE_BACKEND", "gcs")
+		if _, err := Load(); err == nil {
+			t.Fatal("expected error for invalid backend")
+		}
+	})
+
+	t.Run("object url ttl", func(t *testing.T) {
+		setRequiredEnv(t)
+		t.Setenv("STORAGE_BACKEND", "")
+		t.Setenv("OBJECT_URL_TTL", "30m")
+		cfg, err := Load()
+		if err != nil {
+			t.Fatalf("load: %v", err)
+		}
+		if cfg.Storage.URLTTL != 30*time.Minute {
+			t.Fatalf("URLTTL = %v, want 30m", cfg.Storage.URLTTL)
+		}
+	})
+}

backend/internal/photos/helpers_test.go

@@ -100,29 +100,47 @@ func makePNG(t *testing.T) []byte {
 
 // newTestService builds a service backed by the real dev storage. It skips when
 // S3 env is absent so unit runs stay hermetic.
-func newTestService(t *testing.T, pool *pgxpool.Pool) (*Service, *storage.Client) {
+func newTestService(t *testing.T, pool *pgxpool.Pool) (*Service, storage.Backend) {
 	t.Helper()
 	endpoint := os.Getenv("S3_ENDPOINT")
 	if endpoint == "" {
 		t.Skip("S3_ENDPOINT not set")
 	}
-	cl, err := storage.New(context.Background(), config.S3{
+	signer, err := storage.NewSigner("http://app.test", "test-secret", time.Hour)
+	if err != nil {
+		t.Skipf("signer init: %v", err)
+	}
+	cl, err := storage.NewS3(context.Background(), config.S3{
 		Endpoint:     endpoint,
 		Region:       testEnvOr("S3_REGION", "garage"),
 		AccessKey:    os.Getenv("S3_ACCESS_KEY"),
 		SecretKey:    os.Getenv("S3_SECRET_KEY"),
 		Bucket:       testEnvOr("S3_BUCKET", "galo-data"),
 		UsePathStyle: testEnvOr("S3_USE_PATH_STYLE", "true") != "false",
-		PresignTTL:   time.Hour,
-	})
+	}, signer)
 	if err != nil {
 		t.Skipf("storage init: %v", err)
 	}
 	return NewService(NewStore(pool), stubAlbums{}, cl, sse.NewHub(1)), cl
 }
 
+// newTestServiceFS builds a service backed by a temporary local file-system
+// storage backend so the full flow can run without external infrastructure.
+func newTestServiceFS(t *testing.T, pool *pgxpool.Pool) (*Service, storage.Backend) {
+	t.Helper()
+	signer, err := storage.NewSigner("http://app.test", "test-secret", time.Hour)
+	if err != nil {
+		t.Fatalf("storage init: %v", err)
+	}
+	fs, err := storage.NewFS(t.TempDir(), signer)
+	if err != nil {
+		t.Fatalf("storage init: %v", err)
+	}
+	return NewService(NewStore(pool), stubAlbums{}, fs, sse.NewHub(1)), fs
+}
+
 // cleanupPhoto removes a photo, its objects (rows and blobs) and bindings.
-func cleanupPhoto(t *testing.T, ctx context.Context, pool *pgxpool.Pool, cl *storage.Client, photoID string) {
+func cleanupPhoto(t *testing.T, ctx context.Context, pool *pgxpool.Pool, cl storage.Backend, photoID string) {
 	t.Helper()
 	rows, err := pool.Query(ctx,
 		`SELECT o.id, o.storage_key FROM photo_objects po JOIN objects o ON o.id = po.object_id

backend/internal/photos/service.go

@@ -24,11 +24,11 @@ type AlbumAccess interface {
 type Service struct {
 	store   *Store
 	albums  AlbumAccess
-	storage *storage.Client
+	storage storage.Backend
 	hub     *sse.Hub
 }
 
-func NewService(store *Store, albums AlbumAccess, storage *storage.Client, hub *sse.Hub) *Service {
+func NewService(store *Store, albums AlbumAccess, storage storage.Backend, hub *sse.Hub) *Service {
 	return &Service{store: store, albums: albums, storage: storage, hub: hub}
 }
 
@@ -118,7 +118,7 @@ func (s *Service) Upload(ctx context.Context, in UploadInput) (*models.Photo, er
 
 func (s *Service) attachURLs(ctx context.Context, p *models.Photo) error {
 	if p.StorageKey != "" {
-		url, err := s.storage.PresignGet(ctx, p.StorageKey)
+		url, err := s.storage.URL(ctx, p.StorageKey)
 		if err != nil {
 			return err
 		}
@@ -126,12 +126,12 @@ func (s *Service) attachURLs(ctx context.Context, p *models.Photo) error {
 		p.DownloadURL = url
 	}
 	if p.ThumbKey != "" {
-		if url, err := s.storage.PresignGet(ctx, p.ThumbKey); err == nil {
+		if url, err := s.storage.URL(ctx, p.ThumbKey); err == nil {
 			p.ThumbURL = url
 		}
 	}
 	if p.PreviewKey != "" {
-		if url, err := s.storage.PresignGet(ctx, p.PreviewKey); err == nil {
+		if url, err := s.storage.URL(ctx, p.PreviewKey); err == nil {
 			p.PreviewURL = url
 		}
 	}
@@ -202,13 +202,13 @@ func (s *Service) GetPublic(ctx context.Context, photoID string) (*models.Photo,
 	return p, nil
 }
 
-// DownloadPublic returns a presigned original URL for a share link.
+// DownloadPublic returns a signed original URL for a share link.
 func (s *Service) DownloadPublic(ctx context.Context, photoID string) (string, error) {
 	p, err := s.GetPublic(ctx, photoID)
 	if err != nil {
 		return "", err
 	}
-	return s.storage.PresignGet(ctx, p.StorageKey)
+	return s.storage.URL(ctx, p.StorageKey)
 }
 
 func (s *Service) Get(ctx context.Context, photoID, userID string) (*models.Photo, error) {
@@ -236,7 +236,7 @@ func (s *Service) Download(ctx context.Context, photoID, userID string) (string,
 	if err != nil {
 		return "", err
 	}
-	url, err := s.storage.PresignGet(ctx, p.StorageKey)
+	url, err := s.storage.URL(ctx, p.StorageKey)
 	if err != nil {
 		return "", err
 	}

backend/internal/photos/worker_test.go

@@ -6,12 +6,25 @@ import (
 	"time"
 
 	"github.com/jackc/pgx/v5/pgxpool"
+
+	"shared-album/internal/storage"
 )
 
 func TestWorkerGeneratesVariantsAndCollects(t *testing.T) {
 	pool := testPool(t)
-	ctx := context.Background()
 	svc, cl := newTestService(t, pool)
+	runWorkerFlow(t, pool, svc, cl)
+}
+
+func TestWorkerGeneratesVariantsWithFS(t *testing.T) {
+	pool := testPool(t)
+	svc, cl := newTestServiceFS(t, pool)
+	runWorkerFlow(t, pool, svc, cl)
+}
+
+func runWorkerFlow(t *testing.T, pool *pgxpool.Pool, svc *Service, cl storage.Backend) {
+	t.Helper()
+	ctx := context.Background()
 	user := createUser(t, ctx, pool)
 	album := createAlbum(t, ctx, pool, user)
 

backend/internal/storage/fs.go

@@ -0,0 +1,201 @@
+package storage
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"mime"
+	"net/http"
+	"net/url"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// errInvalidKey is returned for object keys that are empty, absolute or attempt
+// to escape the storage root.
+var errInvalidKey = errors.New("fs: invalid object key")
+
+// FS is a Backend backed by a local directory tree. Blobs live under Root at the
+// path formed by the opaque key. Because a local directory has no external URL,
+// FS serves blobs through the app using the shared Signer.
+type FS struct {
+	root   string
+	signer *Signer
+}
+
+// NewFS builds a local file-system storage backend rooted at root. signer issues
+// and validates the app URLs used to serve blobs.
+func NewFS(root string, signer *Signer) (*FS, error) {
+	if strings.TrimSpace(root) == "" {
+		return nil, errors.New("fs: root is required")
+	}
+	if signer == nil {
+		return nil, errors.New("fs: signer is required")
+	}
+	abs, err := filepath.Abs(root)
+	if err != nil {
+		return nil, fmt.Errorf("fs: resolve root: %w", err)
+	}
+	if err := os.MkdirAll(abs, 0o755); err != nil {
+		return nil, fmt.Errorf("fs: create root: %w", err)
+	}
+	return &FS{root: abs, signer: signer}, nil
+}
+
+func (f *FS) Bucket() string { return f.root }
+
+func (f *FS) EnsureBucket(context.Context) error {
+	return os.MkdirAll(f.root, 0o755)
+}
+
+func (f *FS) Ping(context.Context) error {
+	info, err := os.Stat(f.root)
+	if err != nil {
+		return err
+	}
+	if !info.IsDir() {
+		return errors.New("fs: root is not a directory")
+	}
+	return nil
+}
+
+// objectPath maps an opaque key to an absolute path inside the root, rejecting
+// absolute keys and any key that resolves outside the root.
+func (f *FS) objectPath(key string) (string, error) {
+	if key == "" || filepath.IsAbs(key) {
+		return "", errInvalidKey
+	}
+	clean := filepath.Clean(key)
+	if clean == ".." || strings.HasPrefix(clean, ".."+string(os.PathSeparator)) {
+		return "", errInvalidKey
+	}
+	p := filepath.Join(f.root, clean)
+	rel, err := filepath.Rel(f.root, p)
+	if err != nil || rel == ".." || strings.HasPrefix(rel, ".."+string(os.PathSeparator)) {
+		return "", errInvalidKey
+	}
+	return p, nil
+}
+
+func (f *FS) Put(_ context.Context, key string, body io.Reader, _ int64, _ string) error {
+	p, err := f.objectPath(key)
+	if err != nil {
+		return err
+	}
+	if err := os.MkdirAll(filepath.Dir(p), 0o755); err != nil {
+		return fmt.Errorf("fs: mkdir: %w", err)
+	}
+	tmp, err := os.CreateTemp(filepath.Dir(p), ".upload-*")
+	if err != nil {
+		return fmt.Errorf("fs: temp file: %w", err)
+	}
+	tmpName := tmp.Name()
+	defer func() { _ = os.Remove(tmpName) }()
+	if _, err := io.Copy(tmp, body); err != nil {
+		_ = tmp.Close()
+		return fmt.Errorf("fs: write: %w", err)
+	}
+	if err := tmp.Close(); err != nil {
+		return fmt.Errorf("fs: close: %w", err)
+	}
+	if err := os.Rename(tmpName, p); err != nil {
+		return fmt.Errorf("fs: rename: %w", err)
+	}
+	return nil
+}
+
+func (f *FS) Get(_ context.Context, key string) ([]byte, error) {
+	p, err := f.objectPath(key)
+	if err != nil {
+		return nil, err
+	}
+	data, err := os.ReadFile(p)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return nil, ErrObjectNotFound
+		}
+		return nil, err
+	}
+	return data, nil
+}
+
+func (f *FS) Delete(_ context.Context, keys ...string) error {
+	var errs []error
+	for _, key := range keys {
+		p, err := f.objectPath(key)
+		if err != nil {
+			errs = append(errs, err)
+			continue
+		}
+		if err := os.Remove(p); err != nil && !errors.Is(err, os.ErrNotExist) {
+			errs = append(errs, err)
+		}
+	}
+	return errors.Join(errs...)
+}
+
+// URL returns a signed app URL served by this process.
+func (f *FS) URL(_ context.Context, key string) (string, error) {
+	if _, err := f.objectPath(key); err != nil {
+		return "", err
+	}
+	return f.signer.URL(key), nil
+}
+
+// ServeObject validates the signed URL and serves the blob. It supports range
+// requests and sets the content type from the key extension. It writes the
+// response itself and returns nil on success.
+func (f *FS) ServeObject(w http.ResponseWriter, r *http.Request, key string) error {
+	if unescaped, err := url.PathUnescape(key); err == nil {
+		key = unescaped
+	}
+	if err := f.signer.VerifyRequest(key, r); err != nil {
+		return err
+	}
+	p, err := f.objectPath(key)
+	if err != nil {
+		return ErrSignedURLInvalid
+	}
+	file, err := os.Open(p)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return ErrObjectNotFound
+		}
+		return err
+	}
+	defer file.Close()
+	info, err := file.Stat()
+	if err != nil {
+		return err
+	}
+	w.Header().Set("Content-Type", contentTypeForKey(key))
+	http.ServeContent(w, r, info.Name(), info.ModTime(), file)
+	return nil
+}
+
+var imageContentTypes = map[string]string{
+	".jpg":  "image/jpeg",
+	".jpeg": "image/jpeg",
+	".png":  "image/png",
+	".gif":  "image/gif",
+	".webp": "image/webp",
+	".heic": "image/heic",
+	".heif": "image/heif",
+	".tiff": "image/tiff",
+	".tif":  "image/tiff",
+	".bmp":  "image/bmp",
+	".avif": "image/avif",
+}
+
+func contentTypeForKey(key string) string {
+	ext := strings.ToLower(filepath.Ext(key))
+	if ct, ok := imageContentTypes[ext]; ok {
+		return ct
+	}
+	if ct := mime.TypeByExtension(ext); ct != "" {
+		return ct
+	}
+	return "application/octet-stream"
+}

backend/internal/storage/fs_test.go

@@ -0,0 +1,174 @@
+package storage
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+)
+
+func newTestFS(t *testing.T) *FS {
+	t.Helper()
+	signer, err := NewSigner("http://app.test", "test-secret", time.Hour)
+	if err != nil {
+		t.Fatalf("signer: %v", err)
+	}
+	fs, err := NewFS(t.TempDir(), signer)
+	if err != nil {
+		t.Fatalf("new fs: %v", err)
+	}
+	return fs
+}
+
+func TestFSRoundTrip(t *testing.T) {
+	ctx := context.Background()
+	fs := newTestFS(t)
+	key := "objects/abc/original.jpg"
+	data := []byte("hello blob")
+
+	if err := fs.Put(ctx, key, bytes.NewReader(data), int64(len(data)), "image/jpeg"); err != nil {
+		t.Fatalf("put: %v", err)
+	}
+	got, err := fs.Get(ctx, key)
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	if !bytes.Equal(got, data) {
+		t.Fatalf("get = %q, want %q", got, data)
+	}
+	if err := fs.Delete(ctx, key); err != nil {
+		t.Fatalf("delete: %v", err)
+	}
+	if _, err := fs.Get(ctx, key); !errors.Is(err, ErrObjectNotFound) {
+		t.Fatalf("get after delete = %v, want ErrObjectNotFound", err)
+	}
+}
+
+func TestFSPathTraversalRejected(t *testing.T) {
+	ctx := context.Background()
+	fs := newTestFS(t)
+	for _, key := range []string{"../escape", "../../etc/passwd", "/etc/passwd", ""} {
+		if err := fs.Put(ctx, key, strings.NewReader("x"), 1, "text/plain"); err == nil {
+			t.Errorf("put %q: expected rejection", key)
+		}
+		if _, err := fs.Get(ctx, key); err == nil {
+			t.Errorf("get %q: expected rejection", key)
+		}
+		if _, err := fs.URL(ctx, key); err == nil {
+			t.Errorf("url %q: expected rejection", key)
+		}
+	}
+}
+
+func TestFSURLAndServe(t *testing.T) {
+	ctx := context.Background()
+	fs := newTestFS(t)
+	key := "objects/abc/thumb.webp"
+	if err := fs.Put(ctx, key, strings.NewReader("webp-bytes"), 10, "image/webp"); err != nil {
+		t.Fatalf("put: %v", err)
+	}
+
+	raw, err := fs.URL(ctx, key)
+	if err != nil {
+		t.Fatalf("url: %v", err)
+	}
+	u, err := url.Parse(raw)
+	if err != nil {
+		t.Fatalf("parse url: %v", err)
+	}
+	if !strings.HasPrefix(u.Path, "/api/objects/objects/abc/thumb.webp") {
+		t.Fatalf("unexpected path: %s", u.Path)
+	}
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, raw, nil)
+	if err := fs.ServeObject(rec, req, strings.TrimPrefix(u.Path, "/api/objects/")); err != nil {
+		t.Fatalf("serve: %v", err)
+	}
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rec.Code)
+	}
+	if ct := rec.Header().Get("Content-Type"); ct != "image/webp" {
+		t.Fatalf("content-type = %q, want image/webp", ct)
+	}
+	if rec.Body.String() != "webp-bytes" {
+		t.Fatalf("body = %q", rec.Body.String())
+	}
+}
+
+func TestFSServeExpiredAndForged(t *testing.T) {
+	ctx := context.Background()
+	fs := newTestFS(t)
+	key := "objects/abc/original.jpg"
+	if err := fs.Put(ctx, key, strings.NewReader("x"), 1, "image/jpeg"); err != nil {
+		t.Fatalf("put: %v", err)
+	}
+
+	past := time.Now().Add(-time.Minute).Unix()
+	expired := "/api/objects/" + key + "?e=" + strconv.FormatInt(past, 10) + "&s=" + fs.signer.sign(key, past)
+	rec := httptest.NewRecorder()
+	err := fs.ServeObject(rec, httptest.NewRequest(http.MethodGet, expired, nil), key)
+	if !errors.Is(err, ErrSignedURLInvalid) {
+		t.Fatalf("expired = %v, want ErrSignedURLInvalid", err)
+	}
+
+	future := time.Now().Add(time.Hour).Unix()
+	forged := "/api/objects/" + key + "?e=" + strconv.FormatInt(future, 10) + "&s=deadbeef"
+	rec = httptest.NewRecorder()
+	err = fs.ServeObject(rec, httptest.NewRequest(http.MethodGet, forged, nil), key)
+	if !errors.Is(err, ErrSignedURLInvalid) {
+		t.Fatalf("forged = %v, want ErrSignedURLInvalid", err)
+	}
+}
+
+func TestFSServeRangeAndMissing(t *testing.T) {
+	ctx := context.Background()
+	fs := newTestFS(t)
+	key := "objects/abc/original.jpg"
+	if err := fs.Put(ctx, key, strings.NewReader("0123456789"), 10, "image/jpeg"); err != nil {
+		t.Fatalf("put: %v", err)
+	}
+	exp := time.Now().Add(time.Hour).Unix()
+	base := "/api/objects/" + key + "?e=" + strconv.FormatInt(exp, 10) + "&s=" + fs.signer.sign(key, exp)
+
+	req := httptest.NewRequest(http.MethodGet, base, nil)
+	req.Header.Set("Range", "bytes=2-5")
+	rec := httptest.NewRecorder()
+	if err := fs.ServeObject(rec, req, key); err != nil {
+		t.Fatalf("serve range: %v", err)
+	}
+	if rec.Code != http.StatusPartialContent {
+		t.Fatalf("status = %d, want 206", rec.Code)
+	}
+	if body, _ := io.ReadAll(rec.Body); string(body) != "2345" {
+		t.Fatalf("range body = %q, want 2345", body)
+	}
+
+	missingKey := "objects/def/original.jpg"
+	missing := "/api/objects/" + missingKey + "?e=" + strconv.FormatInt(exp, 10) + "&s=" + fs.signer.sign(missingKey, exp)
+	rec = httptest.NewRecorder()
+	err := fs.ServeObject(rec, httptest.NewRequest(http.MethodGet, missing, nil), missingKey)
+	if !errors.Is(err, ErrObjectNotFound) {
+		t.Fatalf("missing = %v, want ErrObjectNotFound", err)
+	}
+}
+
+func TestContentTypeForKey(t *testing.T) {
+	cases := map[string]string{
+		"objects/a/original.jpg":   "image/jpeg",
+		"objects/a/thumb.webp":     "image/webp",
+		"objects/a/odd.unknownext": "application/octet-stream",
+	}
+	for key, want := range cases {
+		if got := contentTypeForKey(key); got != want {
+			t.Errorf("contentTypeForKey(%q) = %q, want %q", key, got, want)
+		}
+	}
+}

backend/internal/storage/s3_test.go

@@ -0,0 +1,142 @@
+package storage
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+
+	"shared-album/internal/config"
+)
+
+func newTestS3(t *testing.T) (*S3, *Signer) {
+	t.Helper()
+	endpoint := os.Getenv("S3_ENDPOINT")
+	if endpoint == "" {
+		t.Skip("S3_ENDPOINT not set")
+	}
+	signer, err := NewSigner("http://app.test", "secret", time.Hour)
+	if err != nil {
+		t.Fatalf("signer: %v", err)
+	}
+	cl, err := NewS3(context.Background(), config.S3{
+		Endpoint:     endpoint,
+		Region:       os.Getenv("S3_REGION"),
+		AccessKey:    os.Getenv("S3_ACCESS_KEY"),
+		SecretKey:    os.Getenv("S3_SECRET_KEY"),
+		Bucket:       os.Getenv("S3_BUCKET"),
+		UsePathStyle: true,
+	}, signer)
+	if err != nil {
+		t.Skipf("storage init: %v", err)
+	}
+	return cl, signer
+}
+
+func signedQuery(signer *Signer, key string, exp int64) string {
+	return "?e=" + strconv.FormatInt(exp, 10) + "&s=" + signer.sign(key, exp)
+}
+
+func TestS3ServeObject(t *testing.T) {
+	ctx := context.Background()
+	cl, signer := newTestS3(t)
+	key := "objects/" + strconv.FormatInt(time.Now().UnixNano(), 10) + "/original.jpg"
+	data := []byte("0123456789")
+	if err := cl.Put(ctx, key, bytes.NewReader(data), int64(len(data)), "image/jpeg"); err != nil {
+		t.Fatalf("put: %v", err)
+	}
+	t.Cleanup(func() { _ = cl.Delete(context.Background(), key) })
+
+	exp := time.Now().Add(time.Hour).Unix()
+	base := "/api/objects/" + key + signedQuery(signer, key, exp)
+
+	t.Run("full", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, base, nil)
+		if err := cl.ServeObject(rec, req, key); err != nil {
+			t.Fatalf("serve: %v", err)
+		}
+		if rec.Code != http.StatusOK {
+			t.Fatalf("status = %d, want 200", rec.Code)
+		}
+		if !bytes.Equal(rec.Body.Bytes(), data) {
+			t.Fatalf("body = %q", rec.Body.String())
+		}
+	})
+
+	t.Run("range", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, base, nil)
+		req.Header.Set("Range", "bytes=2-5")
+		if err := cl.ServeObject(rec, req, key); err != nil {
+			t.Fatalf("serve range: %v", err)
+		}
+		if rec.Code != http.StatusPartialContent {
+			t.Fatalf("status = %d, want 206", rec.Code)
+		}
+		if body, _ := io.ReadAll(rec.Body); string(body) != "2345" {
+			t.Fatalf("range body = %q, want 2345", body)
+		}
+	})
+
+	t.Run("invalid range", func(t *testing.T) {
+		rec := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, base, nil)
+		req.Header.Set("Range", "bytes=1000-2000")
+		err := cl.ServeObject(rec, req, key)
+		if !errors.Is(err, ErrRangeNotSatisfiable) {
+			t.Fatalf("err = %v, want ErrRangeNotSatisfiable", err)
+		}
+	})
+
+	t.Run("missing", func(t *testing.T) {
+		missingKey := "objects/" + strconv.FormatInt(time.Now().UnixNano(), 10) + "/original.jpg"
+		target := "/api/objects/" + missingKey + signedQuery(signer, missingKey, exp)
+		rec := httptest.NewRecorder()
+		err := cl.ServeObject(rec, httptest.NewRequest(http.MethodGet, target, nil), missingKey)
+		if !errors.Is(err, ErrObjectNotFound) {
+			t.Fatalf("err = %v, want ErrObjectNotFound", err)
+		}
+	})
+
+	t.Run("expired signature", func(t *testing.T) {
+		past := time.Now().Add(-time.Minute).Unix()
+		target := "/api/objects/" + key + signedQuery(signer, key, past)
+		rec := httptest.NewRecorder()
+		err := cl.ServeObject(rec, httptest.NewRequest(http.MethodGet, target, nil), key)
+		if !errors.Is(err, ErrSignedURLInvalid) {
+			t.Fatalf("err = %v, want ErrSignedURLInvalid", err)
+		}
+	})
+}
+
+func TestS3ServeObjectHead(t *testing.T) {
+	ctx := context.Background()
+	cl, signer := newTestS3(t)
+	key := "objects/" + strconv.FormatInt(time.Now().UnixNano(), 10) + "/original.jpg"
+	data := []byte("head-check")
+	if err := cl.Put(ctx, key, strings.NewReader(string(data)), int64(len(data)), "image/jpeg"); err != nil {
+		t.Fatalf("put: %v", err)
+	}
+	t.Cleanup(func() { _ = cl.Delete(context.Background(), key) })
+
+	exp := time.Now().Add(time.Hour).Unix()
+	target := "/api/objects/" + key + signedQuery(signer, key, exp)
+	rec := httptest.NewRecorder()
+	if err := cl.ServeObject(rec, httptest.NewRequest(http.MethodHead, target, nil), key); err != nil {
+		t.Fatalf("head: %v", err)
+	}
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rec.Code)
+	}
+	if rec.Body.Len() != 0 {
+		t.Fatalf("HEAD body should be empty, got %d bytes", rec.Body.Len())
+	}
+}

backend/internal/storage/signer.go

@@ -0,0 +1,80 @@
+package storage
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"net/http"
+	"net/url"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// Signer produces and verifies the signed app URLs used by the /api/objects
+// route. It is shared by every storage backend so delivery is identical: the
+// client always talks to the backend, never to object storage directly.
+type Signer struct {
+	secret  []byte
+	baseURL string
+	ttl     time.Duration
+}
+
+// NewSigner builds a signer. secret signs the URLs (required); baseURL is the
+// public base URL of the app; ttl limits URL lifetime (defaults to one hour).
+func NewSigner(baseURL, secret string, ttl time.Duration) (*Signer, error) {
+	if strings.TrimSpace(secret) == "" {
+		return nil, errors.New("url secret is required")
+	}
+	if ttl <= 0 {
+		ttl = time.Hour
+	}
+	return &Signer{
+		secret:  []byte(secret),
+		baseURL: strings.TrimRight(baseURL, "/"),
+		ttl:     ttl,
+	}, nil
+}
+
+// URL returns a signed app URL for the given opaque key.
+func (s *Signer) URL(key string) string {
+	exp := time.Now().Add(s.ttl).Unix()
+	q := url.Values{}
+	q.Set("e", strconv.FormatInt(exp, 10))
+	q.Set("s", s.sign(key, exp))
+	return fmt.Sprintf("%s/api/objects/%s?%s", s.baseURL, escapeKey(key), q.Encode())
+}
+
+// VerifyRequest checks the signature and expiry carried by an object request.
+// It returns ErrSignedURLInvalid when the request is not authorized.
+func (s *Signer) VerifyRequest(key string, r *http.Request) error {
+	q := r.URL.Query()
+	exp, err := strconv.ParseInt(q.Get("e"), 10, 64)
+	if err != nil || !s.verify(key, exp, q.Get("s")) {
+		return ErrSignedURLInvalid
+	}
+	return nil
+}
+
+func (s *Signer) sign(key string, exp int64) string {
+	mac := hmac.New(sha256.New, s.secret)
+	_, _ = fmt.Fprintf(mac, "%s\n%d", key, exp)
+	return hex.EncodeToString(mac.Sum(nil))
+}
+
+func (s *Signer) verify(key string, exp int64, sig string) bool {
+	if sig == "" || time.Now().Unix() > exp {
+		return false
+	}
+	return hmac.Equal([]byte(s.sign(key, exp)), []byte(sig))
+}
+
+func escapeKey(key string) string {
+	parts := strings.Split(key, "/")
+	for i, p := range parts {
+		parts[i] = url.PathEscape(p)
+	}
+	return strings.Join(parts, "/")
+}

backend/internal/storage/storage.go

@@ -5,8 +5,9 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"net/http"
+	"strconv"
 	"strings"
-	"time"
 
 	"github.com/aws/aws-sdk-go-v2/aws"
 	awsconfig "github.com/aws/aws-sdk-go-v2/config"
@@ -18,14 +19,49 @@ import (
 	"shared-album/internal/config"
 )
 
-type Client struct {
-	s3         *s3.Client
-	presign    *s3.PresignClient
-	bucket     string
-	presignTTL time.Duration
+// ErrSignedURLInvalid is returned when an app-served object URL has a missing,
+// malformed, expired or forged signature.
+var ErrSignedURLInvalid = errors.New("invalid or expired object signature")
+
+// ErrObjectNotFound is returned when the requested blob does not exist.
+var ErrObjectNotFound = errors.New("object not found")
+
+// ErrRangeNotSatisfiable is returned when a requested byte range is invalid.
+var ErrRangeNotSatisfiable = errors.New("requested range not satisfiable")
+
+// Backend is the object storage abstraction. Keys are opaque and independent of
+// the backend: every implementation stores and serves the same logical blob.
+// Blobs are always delivered through the app.
+type Backend interface {
+	Put(ctx context.Context, key string, body io.Reader, size int64, contentType string) error
+	Delete(ctx context.Context, keys ...string) error
+	Get(ctx context.Context, key string) ([]byte, error)
+	// URL returns a time-limited signed app URL the client can use to fetch the blob.
+	URL(ctx context.Context, key string) (string, error)
+	// ServeObject validates the signed URL and writes the blob, including
+	// Content-Type and range handling, to the response.
+	ServeObject(w http.ResponseWriter, r *http.Request, key string) error
+	Ping(ctx context.Context) error
+	EnsureBucket(ctx context.Context) error
+	// Bucket returns a human-readable identifier for the storage location.
+	Bucket() string
 }
 
-func New(ctx context.Context, cfg config.S3) (*Client, error) {
+// S3 is a Backend backed by an S3-compatible object store (Garage). Blobs are
+// delivered through the app: the client receives a signed app URL and the bytes
+// are streamed from S3 over the internal network.
+type S3 struct {
+	s3     *s3.Client
+	bucket string
+	signer *Signer
+}
+
+// NewS3 builds an S3-compatible storage backend. signer issues the app URLs used
+// to serve blobs.
+func NewS3(ctx context.Context, cfg config.S3, signer *Signer) (*S3, error) {
+	if signer == nil {
+		return nil, errors.New("s3: signer is required")
+	}
 	awsCfg, err := awsconfig.LoadDefaultConfig(ctx,
 		awsconfig.WithRegion(cfg.Region),
 		awsconfig.WithCredentialsProvider(credentials.NewStaticCredentialsProvider(
@@ -41,17 +77,38 @@ func New(ctx context.Context, cfg config.S3) (*Client, error) {
 		o.UsePathStyle = cfg.UsePathStyle
 	})
 
-	return &Client{
-		s3:         client,
-		presign:    s3.NewPresignClient(client),
-		bucket:     cfg.Bucket,
-		presignTTL: cfg.PresignTTL,
+	return &S3{
+		s3:     client,
+		bucket: cfg.Bucket,
+		signer: signer,
 	}, nil
 }
 
-func (c *Client) Bucket() string { return c.bucket }
+func (c *S3) Bucket() string { return c.bucket }
 
-func (c *Client) EnsureBucket(ctx context.Context) error {
+// Open builds the storage backend selected by configuration: S3-compatible by
+// default, or the local file system when STORAGE_BACKEND=fs. Every backend
+// delivers blobs through the app using a shared Signer.
+func Open(ctx context.Context, cfg *config.Config) (Backend, error) {
+	secret := cfg.Storage.URLSecret
+	if secret == "" {
+		secret = cfg.Auth.JWTSecret
+	}
+	signer, err := NewSigner(cfg.Server.BaseURL, secret, cfg.Storage.URLTTL)
+	if err != nil {
+		return nil, err
+	}
+	switch cfg.Storage.Backend {
+	case "", "s3":
+		return NewS3(ctx, cfg.S3, signer)
+	case "fs":
+		return NewFS(cfg.Storage.FSRoot, signer)
+	default:
+		return nil, fmt.Errorf("unknown storage backend %q", cfg.Storage.Backend)
+	}
+}
+
+func (c *S3) EnsureBucket(ctx context.Context) error {
 	_, err := c.s3.HeadBucket(ctx, &s3.HeadBucketInput{Bucket: aws.String(c.bucket)})
 	if err == nil {
 		return nil
@@ -66,12 +123,12 @@ func (c *Client) EnsureBucket(ctx context.Context) error {
 	return nil
 }
 
-func (c *Client) Ping(ctx context.Context) error {
+func (c *S3) Ping(ctx context.Context) error {
 	_, err := c.s3.HeadBucket(ctx, &s3.HeadBucketInput{Bucket: aws.String(c.bucket)})
 	return err
 }
 
-func (c *Client) Put(ctx context.Context, key string, body io.Reader, size int64, contentType string) error {
+func (c *S3) Put(ctx context.Context, key string, body io.Reader, size int64, contentType string) error {
 	in := &s3.PutObjectInput{
 		Bucket:      aws.String(c.bucket),
 		Key:         aws.String(key),
@@ -87,7 +144,7 @@ func (c *Client) Put(ctx context.Context, key string, body io.Reader, size int64
 	return nil
 }
 
-func (c *Client) Delete(ctx context.Context, keys ...string) error {
+func (c *S3) Delete(ctx context.Context, keys ...string) error {
 	if len(keys) == 0 {
 		return nil
 	}
@@ -104,7 +161,7 @@ func (c *Client) Delete(ctx context.Context, keys ...string) error {
 	return nil
 }
 
-func (c *Client) Get(ctx context.Context, key string) ([]byte, error) {
+func (c *S3) Get(ctx context.Context, key string) ([]byte, error) {
 	out, err := c.s3.GetObject(ctx, &s3.GetObjectInput{
 		Bucket: aws.String(c.bucket),
 		Key:    aws.String(key),
@@ -120,14 +177,97 @@ func (c *Client) Get(ctx context.Context, key string) ([]byte, error) {
 	return data, nil
 }
 
-func (c *Client) PresignGet(ctx context.Context, key string) (string, error) {	out, err := c.presign.PresignGetObject(ctx, &s3.GetObjectInput{
+// URL returns a signed app URL. The blob itself is streamed by ServeObject, so
+// the S3 endpoint is never exposed to the client.
+func (c *S3) URL(_ context.Context, key string) (string, error) {
+	return c.signer.URL(key), nil
+}
+
+// ServeObject validates the signed URL and streams the blob from S3. It passes
+// the client's Range header through to S3, so byte ranges are handled natively,
+// and answers HEAD via HeadObject. It writes the response itself.
+func (c *S3) ServeObject(w http.ResponseWriter, r *http.Request, key string) error {
+	if err := c.signer.VerifyRequest(key, r); err != nil {
+		return err
+	}
+	if r.Method == http.MethodHead {
+		return c.headObject(w, r, key)
+	}
+	in := &s3.GetObjectInput{Bucket: aws.String(c.bucket), Key: aws.String(key)}
+	if rng := r.Header.Get("Range"); rng != "" {
+		in.Range = aws.String(rng)
+	}
+	out, err := c.s3.GetObject(r.Context(), in)
+	if err != nil {
+		if isNoSuchKey(err) {
+			return ErrObjectNotFound
+		}
+		if isInvalidRange(err) {
+			return ErrRangeNotSatisfiable
+		}
+		return err
+	}
+	defer out.Body.Close()
+	if ct := aws.ToString(out.ContentType); ct != "" {
+		w.Header().Set("Content-Type", ct)
+	} else {
+		w.Header().Set("Content-Type", contentTypeForKey(key))
+	}
+	if out.ContentLength != nil {
+		w.Header().Set("Content-Length", strconv.FormatInt(*out.ContentLength, 10))
+	}
+	if out.ContentRange != nil {
+		w.Header().Set("Content-Range", *out.ContentRange)
+	}
+	status := http.StatusOK
+	if out.ContentRange != nil {
+		status = http.StatusPartialContent
+	}
+	w.WriteHeader(status)
+	if _, err := io.Copy(w, out.Body); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (c *S3) headObject(w http.ResponseWriter, r *http.Request, key string) error {
+	out, err := c.s3.HeadObject(r.Context(), &s3.HeadObjectInput{
 		Bucket: aws.String(c.bucket),
 		Key:    aws.String(key),
-	}, s3.WithPresignExpires(c.presignTTL))
+	})
 	if err != nil {
-		return "", fmt.Errorf("presign %s: %w", key, err)
+		if isNoSuchKey(err) {
+			return ErrObjectNotFound
+		}
+		return err
 	}
-	return out.URL, nil
+	if ct := aws.ToString(out.ContentType); ct != "" {
+		w.Header().Set("Content-Type", ct)
+	} else {
+		w.Header().Set("Content-Type", contentTypeForKey(key))
+	}
+	if out.ContentLength != nil {
+		w.Header().Set("Content-Length", strconv.FormatInt(*out.ContentLength, 10))
+	}
+	w.WriteHeader(http.StatusOK)
+	return nil
+}
+
+func isNoSuchKey(err error) bool {
+	var apiErr smithy.APIError
+	if errors.As(err, &apiErr) {
+		code := apiErr.ErrorCode()
+		return strings.EqualFold(code, "NoSuchKey") || strings.EqualFold(code, "NotFound")
+	}
+	return false
+}
+
+func isInvalidRange(err error) bool {
+	var apiErr smithy.APIError
+	if errors.As(err, &apiErr) {
+		return strings.EqualFold(apiErr.ErrorCode(), "InvalidRange")
+	}
+	return false
 }
 
 func isNoSuchBucket(err error) bool {