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/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, "/")
+}