Purge unused genres at the end of the scan

persistence/persistence.go

@@ -165,6 +165,11 @@ func (s *SQLStore) GC(ctx context.Context, rootFolder string) error {
 	if err != nil {
 		log.Error(ctx, "Error tidying up playlists", err)
 	}
+	err = s.Genre(ctx).(*genreRepository).purgeEmpty()
+	if err != nil {
+		log.Error(ctx, "Error removing unused genres", err)
+		return err
+	}
 	return err
 }