Add photo content-hash dedup and local↔S3 library sync.

Uploads skip per-gallery duplicates, checksums can be backfilled, and STORAGE_BACKEND plus sync tooling make switching storage backends safe.

photo-api/internal/storage/local.go

@@ -17,6 +17,12 @@ type local struct {
 	root string
 }
 
+// NewLocal builds the local-disk backend explicitly, whichever backend is
+// active — `photo-api sync` needs both sides at once.
+func NewLocal(root string) (Storage, error) {
+	return newLocal(root)
+}
+
 func newLocal(root string) (*local, error) {
 	if err := os.MkdirAll(root, 0o755); err != nil {
 		return nil, fmt.Errorf("create storage dir %s: %w", root, err)
@@ -74,6 +80,21 @@ func (l *local) Open(_ context.Context, key string) (io.ReadCloser, int64, error
 	return f, info.Size(), nil
 }
 
+func (l *local) Stat(_ context.Context, key string) (int64, error) {
+	p, err := l.path(key)
+	if err != nil {
+		return 0, err
+	}
+	info, err := os.Stat(p)
+	if os.IsNotExist(err) {
+		return 0, ErrNotExist
+	}
+	if err != nil {
+		return 0, err
+	}
+	return info.Size(), nil
+}
+
 func (l *local) Delete(_ context.Context, key string) error {
 	p, err := l.path(key)
 	if err != nil {