feat(photos): separate object storage from metadata layer

Move physical file facts out of the photos table into an objects ledger:

- objects / photo_objects / variant_jobs tables (migration 000008)
- drop storage_key/thumb_key/preview_key/size/width/height/mime_type from
  photos (migration 000009); they are derived via joins to the original object
- album-free storage keys: objects/{id}/{role}.{ext}
- explicit GC via pending/ready/deleting states; objects deleted only when no
  photo references them
- background worker drives variant generation (durable variant_jobs) and GC
- copy/move are metadata-only; storage objects are shared between photos
- remove unused storage.DeletePrefix

Non-destructive to the external JSON API: size/dimensions/mime and URLs are
still returned, just sourced from the object layer.

OpenSpec change: media-object-layer

backend/cmd/server/main.go

@@ -69,6 +69,8 @@ func main() {
 	hub := sse.NewHub(cfg.SSE.ClientBuffer)
 	server := app.New(cfg, pool, store, hub)
 
+	go server.StartWorker(ctx)
+
 	httpServer := &http.Server{
 		Addr:              cfg.Server.Addr,
 		Handler:           server.Router(),