fix(photos): faster large-album loading and album cover lifecycle

Large albums opened slowly and flickered; the default album cover was
also lost in the media-objects refactor.

Large-album loading:
- quantize signed object URLs within a window and send Cache-Control so
  previews are reused instead of re-fetched
- publish photo.updated (not photo.created) when variants are ready, so
  clients update in place without reordering the grid
- worker concurrency + newest-first variant priority; raise default batch

Album cover:
- first uploaded photo becomes the cover when none is set
- clear the cover when the cover photo is trashed or moved out
- "set as album cover" in the photo context menu and actions panel

OpenSpec changes: fix-large-album-load, fix-album-cover

backend/internal/photos/objects.go

@@ -296,11 +296,15 @@ func (s *Store) ClaimVariantJobs(ctx context.Context, limit int, lease time.Dura
 	defer func() { _ = tx.Rollback(ctx) }()
 
 	rows, err := tx.Query(ctx,
-		`SELECT photo_id FROM variant_jobs
-		 WHERE next_attempt_at <= now()
-		 ORDER BY next_attempt_at
+		`SELECT vj.photo_id FROM variant_jobs vj
+		 JOIN photos p ON p.id = vj.photo_id
+		 WHERE vj.next_attempt_at <= now()
+		 ORDER BY (vj.attempts > 0) ASC,
+		          CASE WHEN vj.attempts = 0 THEN p.created_at END DESC NULLS LAST,
+		          vj.next_attempt_at ASC,
+		          vj.photo_id DESC
 		 LIMIT $1
-		 FOR UPDATE SKIP LOCKED`, limit)
+		 FOR UPDATE OF vj SKIP LOCKED`, limit)
 	if err != nil {
 		return nil, err
 	}