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/store.go

@@ -424,6 +424,14 @@ func (s *Store) SetCoverIfEmpty(ctx context.Context, albumID, photoID string) {
 		albumID, photoID)
 }
 
+// ClearCoverIfMatches removes the album cover only when it currently points at
+// photoID, so a concurrent cover change is not clobbered.
+func (s *Store) ClearCoverIfMatches(ctx context.Context, albumID, photoID string) {
+	_, _ = s.pool.Exec(ctx,
+		`UPDATE albums SET cover_photo_id = NULL WHERE id = $1 AND cover_photo_id = $2`,
+		albumID, photoID)
+}
+
 func (s *Store) UpdateFilename(ctx context.Context, id, filename string) error {
 	tag, err := s.pool.Exec(ctx, `UPDATE photos SET original_filename = $2 WHERE id = $1`, id, filename)
 	if err != nil {