Expose OpenSubsonic fields Genres, MusicBrainzId, Bpm, Comment (#2597)

* add Genres to subsonic responses

* add genres in GetAlbum response

* add musicBrainzId

* add Bpm and Comment OpenSubsonic fields

* remove omitempty on OpenSubsonic fields

* add custom JSON marshalers to ensure genres attribute is non-nil

* regenerate snapshots to capture now-mandatory fields

server/subsonic/browsing.go

@@ -423,6 +423,7 @@ func (api *Router) buildAlbum(ctx context.Context, album *model.Album, mfs model
 	}
 	dir.Year = int32(album.MaxYear)
 	dir.Genre = album.Genre
+	dir.Genres = itemGenresFromGenres(album.Genres)
 	dir.UserRating = int32(album.Rating)
 	if !album.CreatedAt.IsZero() {
 		dir.Created = &album.CreatedAt
@@ -430,6 +431,7 @@ func (api *Router) buildAlbum(ctx context.Context, album *model.Album, mfs model
 	if album.Starred {
 		dir.Starred = &album.StarredAt
 	}
+	dir.MusicBrainzId = album.MbzAlbumID
 	dir.Song = childrenFromMediaFiles(ctx, mfs)
 	return dir
 }