feat: implement shared-album multi-user photo album

Backend (Go/Gin/pgx): env config, pgx pool with retries, Garage S3 client
(path-style, region garage), embedded golang-migrate migrations, JWT auth with
refresh rotation, albums CRUD, members/roles/invites with no-escalation matrix,
photo upload + async imaging/goexif WebP variants, cursor pagination, presigned
URLs, likes/comments, SSE hub (album + personal), notifications, share links.

Frontend (SvelteKit 2 + Svelte 5 + Tailwind v4): API wrapper with 401 refresh,
auth store, all screens, Uploader/PhotoGrid/Lightbox/Comments, SSE client with
reconnect + REST catch-up, three themes + system via data-theme.

Deploy without Docker: Makefile builds the frontend into backend/static and
embeds it (go:embed) into a single statically linked binary; systemd unit and
nginx SSE example included.

Use gen2brain/webp (-tags nodynamic) instead of cgo kolesa-team/go-webp to keep
CGO_ENABLED=0 static builds; design.md/proposal.md updated accordingly.

backend/internal/sharelinks/service.go

@@ -0,0 +1,95 @@
+package sharelinks
+
+import (
+	"context"
+	"errors"
+	"time"
+
+	"shared-album/internal/auth"
+	"shared-album/internal/httpx"
+	"shared-album/internal/models"
+	"shared-album/internal/roles"
+)
+
+type AlbumAccess interface {
+	Access(ctx context.Context, albumID, userID string) (*models.Album, string, error)
+}
+
+type Service struct {
+	store  *Store
+	albums AlbumAccess
+}
+
+func NewService(store *Store, albums AlbumAccess) *Service {
+	return &Service{store: store, albums: albums}
+}
+
+func (s *Service) Create(ctx context.Context, albumID, actorID string, ttl time.Duration) (*models.ShareLink, error) {
+	_, role, err := s.albums.Access(ctx, albumID, actorID)
+	if err != nil {
+		return nil, err
+	}
+	if !roles.CanManageShares(role) {
+		return nil, httpx.Forbidden("forbidden", "only owner and admin can manage share links")
+	}
+	if ttl <= 0 {
+		ttl = 7 * 24 * time.Hour
+	}
+	token, _, err := auth.NewRefreshToken()
+	if err != nil {
+		return nil, err
+	}
+	l := &models.ShareLink{
+		ID:        auth.NewID(),
+		AlbumID:   albumID,
+		Token:     token,
+		CreatedBy: actorID,
+		ExpiresAt: time.Now().Add(ttl),
+	}
+	if err := s.store.Create(ctx, l); err != nil {
+		return nil, err
+	}
+	return l, nil
+}
+
+func (s *Service) List(ctx context.Context, albumID, actorID string) ([]models.ShareLink, error) {
+	_, role, err := s.albums.Access(ctx, albumID, actorID)
+	if err != nil {
+		return nil, err
+	}
+	if !roles.CanManageShares(role) {
+		return nil, httpx.Forbidden("forbidden", "only owner and admin can manage share links")
+	}
+	return s.store.ListByAlbum(ctx, albumID)
+}
+
+func (s *Service) Revoke(ctx context.Context, albumID, linkID, actorID string) error {
+	_, role, err := s.albums.Access(ctx, albumID, actorID)
+	if err != nil {
+		return err
+	}
+	if !roles.CanManageShares(role) {
+		return httpx.Forbidden("forbidden", "only owner and admin can manage share links")
+	}
+	if err := s.store.Revoke(ctx, linkID, albumID); err != nil {
+		if errors.Is(err, ErrNotFound) {
+			return httpx.NotFound("share_link_not_found", "share link not found")
+		}
+		return err
+	}
+	return nil
+}
+
+func (s *Service) Resolve(ctx context.Context, token string) (*models.ShareLink, error) {
+	l, err := s.store.GetByToken(ctx, token)
+	if err != nil {
+		if errors.Is(err, ErrNotFound) {
+			return nil, httpx.NotFound("share_link_not_found", "share link not found")
+		}
+		return nil, err
+	}
+	if !l.Active(time.Now()) {
+		return nil, httpx.Forbidden("share_link_inactive", "share link is expired or revoked")
+	}
+	return l, nil
+}