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

internal/service/metadata.go

@@ -108,6 +108,8 @@ func (s *MetadataService) GetAlbum(ctx context.Context, id string) (*domain.Albu
 		return nil, err
 	}
 
+	s.ensureArtistsCached(ctx, []domain.Album{*album})
+
 	if saveErr := s.albums.Save(ctx, album); saveErr != nil {
 		log.Warn().Err(saveErr).Str("album_id", id).Msg("failed to cache album")
 		return album, nil
@@ -246,5 +248,20 @@ func (s *MetadataService) GetAlbumTracks(ctx context.Context, albumID string) ([
 
 	metrics.CacheMisses.WithLabelValues("album_tracks").Inc()
 	log.Debug().Str("album_id", albumID).Str("provider", s.provider.Name()).Msg("album tracks cache miss, querying provider")
-	return s.provider.GetAlbumTracks(ctx, albumID)
+
+	tracks, err = s.provider.GetAlbumTracks(ctx, albumID)
+	if err != nil {
+		return nil, err
+	}
+
+	album, err := s.albums.GetByExternalID(ctx, s.provider.Name(), albumID)
+	if err == nil {
+		if saveErr := s.tracks.SaveAlbumTracks(ctx, album.ID, tracks); saveErr != nil {
+			log.Warn().Err(saveErr).Str("album_id", albumID).Msg("failed to cache album tracks")
+		} else {
+			log.Debug().Str("album_id", albumID).Int("count", len(tracks)).Msg("album tracks cached")
+		}
+	}
+
+	return tracks, nil
 }