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/artist_settings.go

@@ -1,28 +1,33 @@
 package database
 
 import (
+	"database/sql"
 	"fmt"
 )
 
 // GetArtistSettings retrieves an artist_settings row by ID.
 // Returns sql.ErrNoRows if the artist is not found.
 func GetArtistSettings(db *DB, id string) (*ArtistSettings, error) {
-	var s ArtistSettings
+	var (
+		s    ArtistSettings
+		mbid sql.NullString
+	)
 	err := db.Conn().QueryRow(
-		"SELECT id, name, ignore_singles, ignore_compilations, monitored FROM artist_settings WHERE id = ?",
+		"SELECT id, name, mbid, ignore_singles, ignore_compilations, monitored FROM artist_settings WHERE id = ?",
 		id,
-	).Scan(&s.ID, &s.Name, &s.IgnoreSingles, &s.IgnoreCompilations, &s.Monitored)
+	).Scan(&s.ID, &s.Name, &mbid, &s.IgnoreSingles, &s.IgnoreCompilations, &s.Monitored)
 	if err != nil {
 		return nil, err
 	}
+	s.MBID = mbid.String
 	return &s, nil
 }
 
 // SaveArtistSettings inserts or replaces an artist_settings row.
 func SaveArtistSettings(db *DB, settings *ArtistSettings) error {
 	_, err := db.Conn().Exec(
-		"INSERT OR REPLACE INTO artist_settings (id, name, ignore_singles, ignore_compilations, monitored) VALUES (?, ?, ?, ?, ?)",
-		settings.ID, settings.Name, settings.IgnoreSingles, settings.IgnoreCompilations, settings.Monitored,
+		"INSERT OR REPLACE INTO artist_settings (id, name, mbid, ignore_singles, ignore_compilations, monitored) VALUES (?, ?, ?, ?, ?, ?)",
+		settings.ID, settings.Name, settings.MBID, settings.IgnoreSingles, settings.IgnoreCompilations, settings.Monitored,
 	)
 	if err != nil {
 		return fmt.Errorf("save artist settings: %w", err)
@@ -33,7 +38,7 @@ func SaveArtistSettings(db *DB, settings *ArtistSettings) error {
 // GetAllArtistSettings returns all rows from artist_settings.
 func GetAllArtistSettings(db *DB) ([]ArtistSettings, error) {
 	rows, err := db.Conn().Query(
-		"SELECT id, name, ignore_singles, ignore_compilations, monitored FROM artist_settings",
+		"SELECT id, name, mbid, ignore_singles, ignore_compilations, monitored FROM artist_settings",
 	)
 	if err != nil {
 		return nil, fmt.Errorf("query all artist settings: %w", err)
@@ -43,7 +48,7 @@ func GetAllArtistSettings(db *DB) ([]ArtistSettings, error) {
 	var results []ArtistSettings
 	for rows.Next() {
 		var s ArtistSettings
-		if err := rows.Scan(&s.ID, &s.Name, &s.IgnoreSingles, &s.IgnoreCompilations, &s.Monitored); err != nil {
+		if err := rows.Scan(&s.ID, &s.Name, &s.MBID, &s.IgnoreSingles, &s.IgnoreCompilations, &s.Monitored); err != nil {
 			return nil, fmt.Errorf("scan artist settings: %w", err)
 		}
 		results = append(results, s)
@@ -74,6 +79,12 @@ func UpdateArtistSettings(db *DB, id string, updates map[string]interface{}) err
 			}
 			setClause += "name = ?"
 			args = append(args, val)
+		case "mbid":
+			if setClause != "" {
+				setClause += ", "
+			}
+			setClause += "mbid = ?"
+			args = append(args, val)
 		case "ignore_singles":
 			if setClause != "" {
 				setClause += ", "