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.

frontend/src/lib/api/photos.ts

@@ -52,6 +52,9 @@ export interface Photo {
     preview?: string;
     original: string;
   };
+  // Upload responses only: these bytes were already in the gallery, so nothing
+  // was stored and the rest of this object describes the existing photo.
+  duplicate?: boolean;
 }
 
 export interface CreateGalleryInput {