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/internal/albums/store.go

@@ -44,11 +44,12 @@ func (s *Store) GetByID(ctx context.Context, id string) (*models.Album, error) {
 	var a models.Album
 	err := s.pool.QueryRow(ctx,
 		`SELECT a.id, a.owner_id, a.title, a.description, a.cover_photo_id, a.created_at, a.updated_at,
-		        COALESCE(p.thumb_key, ''),
+		        COALESCE(co.storage_key, ''),
 		        (SELECT count(*) FROM photos WHERE album_id = a.id),
 		        (SELECT count(*) FROM album_members WHERE album_id = a.id)
 		 FROM albums a
-		 LEFT JOIN photos p ON p.id = a.cover_photo_id
+		 LEFT JOIN photo_objects cpo ON cpo.photo_id = a.cover_photo_id AND cpo.role = 'thumb'
+		 LEFT JOIN objects co ON co.id = cpo.object_id
 		 WHERE a.id = $1`, id).
 		Scan(&a.ID, &a.OwnerID, &a.Title, &a.Description, &a.CoverPhotoID, &a.CreatedAt, &a.UpdatedAt, &a.CoverThumbKey, &a.PhotosCount, &a.MembersCount)
 	if err != nil {
@@ -63,12 +64,13 @@ func (s *Store) GetByID(ctx context.Context, id string) (*models.Album, error) {
 func (s *Store) ListForUser(ctx context.Context, userID string) ([]models.Album, error) {
 	rows, err := s.pool.Query(ctx,
 		`SELECT a.id, a.owner_id, a.title, a.description, a.cover_photo_id, a.created_at, a.updated_at,
-		        COALESCE(p.thumb_key, ''),
+		        COALESCE(co.storage_key, ''),
 		        (SELECT count(*) FROM photos WHERE album_id = a.id),
 		        (SELECT count(*) FROM album_members WHERE album_id = a.id)
 		 FROM albums a
 		 LEFT JOIN album_members m ON m.album_id = a.id AND m.user_id = $1
-		 LEFT JOIN photos p ON p.id = a.cover_photo_id
+		 LEFT JOIN photo_objects cpo ON cpo.photo_id = a.cover_photo_id AND cpo.role = 'thumb'
+		 LEFT JOIN objects co ON co.id = cpo.object_id
 		 WHERE a.owner_id = $1 OR m.user_id IS NOT NULL
 		 ORDER BY a.created_at DESC`, userID)
 	if err != nil {