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.go

@@ -125,6 +125,10 @@ func (db *DB) migrate() error {
 			name: "007_index_external_releases_artist_id",
 			sql:  `CREATE INDEX IF NOT EXISTS idx_external_releases_artist_id ON external_releases(artist_id);`,
 		},
+		{
+			name: "008_add_mbid_to_artist_settings",
+			sql:  `ALTER TABLE artist_settings ADD COLUMN mbid TEXT;`,
+		},
 	}
 
 	for _, m := range migrations {
@@ -173,6 +177,7 @@ func (db *DB) isMigrationApplied(name string) (bool, error) {
 type ArtistSettings struct {
 	ID                 string `json:"id"`
 	Name               string `json:"name"`
+	MBID               string `json:"mbid"`
 	IgnoreSingles      bool   `json:"ignore_singles"`
 	IgnoreCompilations bool   `json:"ignore_compilations"`
 	Monitored          bool   `json:"monitored"`