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/httpapi/dto.go

@@ -58,6 +58,10 @@ type photoJSON struct {
 	LastError        string    `json:"lastError,omitempty"` // admin only
 	CreatedAt        string    `json:"createdAt"`
 	URLs             photoURLs `json:"urls"`
+	// Duplicate marks an upload that was skipped because the gallery already
+	// held these bytes; the rest of the object describes the existing photo.
+	// Set by the upload handler only, never persisted.
+	Duplicate bool `json:"duplicate,omitempty"`
 }
 
 // viewTokenFor returns the token to embed in a gallery's file URLs: none