feat: wire main loop with periodic sync+scan and sync_interval config

Replace compute-only run() with an immediate sync+scan followed by a
ticker-driven periodic loop, add the sync.interval config field (default
6h) with defaults and validation, and add tests for the scheduling logic.

internal/musicbrainz/syncall.go

@@ -58,6 +58,12 @@ func (s *albumSyncer) SyncAlbums(ctx context.Context, db *database.DB) error {
 	return s.syncAlbums(ctx, db)
 }
 
+// NewAlbumSyncer adapts the given SyncAlbums function (typically
+// navidrome.SyncAlbums) into an AlbumSyncer for injection into SyncAll.
+func NewAlbumSyncer(syncAlbums func(ctx context.Context, db *database.DB) error) AlbumSyncer {
+	return &albumSyncer{syncAlbums: syncAlbums}
+}
+
 // SyncAll orchestrates the data pipeline for every monitored artist:
 //  1. MusicBrainz artist-ID resolution — for each artist with no cached MBID,
 //     resolve it by name and persist it on the artist_settings row. Artists that