feat: cache album tracks and load album artists on album fetch

- Add SaveAlbumTracks to TrackRepository for batch track caching
- Extend album loadRelations to join album_artists with artist data
- Cache album tracks in GetAlbumTracks after provider fetch
- Ensure artists cached when fetching single album

cmd/server/noop_repo.go

@@ -72,3 +72,7 @@ func (r *noopTrackRepo) GetByAlbumID(ctx context.Context, albumID string) ([]dom
 func (r *noopTrackRepo) Save(ctx context.Context, track *domain.Track) error {
 	return nil
 }
+
+func (r *noopTrackRepo) SaveAlbumTracks(ctx context.Context, albumID string, tracks []domain.Track) error {
+	return nil
+}