feat: add mbid column to artist_settings

Additive migration 008 adds an mbid column to artist_settings, extend the
ArtistSettings struct and persistence functions (SaveArtistSettings,
GetArtistSettings, GetAllArtistSettings, UpdateArtistSettings) to carry the
MusicBrainz ID, and add round-trip tests covering empty and set MBID values.

internal/database/database_test.go

@@ -205,10 +205,11 @@ func TestMigrationTracking(t *testing.T) {
 		t.Fatalf("query migrations count: %v", err)
 	}
 
-	// We have 7 recorded migrations: artist_settings, external_releases,
+	// We have 8 recorded migrations: artist_settings, external_releases,
 	// local_albums, notifications_sent, cached_at column, secondary_types
-	// column, and the external_releases.artist_id index.
-	if count != 7 {
-		t.Errorf("expected 7 applied migrations, got %d", count)
+	// column, the external_releases.artist_id index, and the artist_settings
+	// mbid column.
+	if count != 8 {
+		t.Errorf("expected 8 applied migrations, got %d", count)
 	}
 }