feat: migration 13 — add MBID columns to artists, release_groups, recordings

Add nullable TEXT mbid column to artists, release_groups, and
recordings tables. Partial indexes on each (WHERE mbid IS NOT NULL)
for fast MBID lookups without bloating the index for rows without
MBIDs.

Enables linking local library entities to MusicBrainz/ListenBrainz
explore data, artist image sharing, and 'In Library' badges.

backend/database/database.go

@@ -370,6 +370,16 @@ func runMigrations(
 		}
 	}
 
+	// Migration 13: add MusicBrainz ID columns to artists,
+	// release_groups, and recordings for library↔explore linking.
+	if version < 13 { //nolint:mnd
+		if err := migration13MBIDColumns(
+			ctx, db, logger,
+		); err != nil {
+			return err
+		}
+	}
+
 	return nil
 }
 
@@ -1530,6 +1540,62 @@ func migration12ExploreSearchIndex(
 	return nil
 }
 
+// migration13MBIDColumns adds MusicBrainz ID columns to artists,
+// release_groups, and recordings for linking local library entities
+// to MusicBrainz/ListenBrainz explore data.
+func migration13MBIDColumns(
+	ctx context.Context,
+	db *sql.DB,
+	logger *slog.Logger,
+) error {
+	logger.Info("applying migration 13: MusicBrainz ID columns")
+
+	alterStmts := []struct {
+		table  string
+		column string
+	}{
+		{"artists", "mbid"},
+		{"release_groups", "mbid"},
+		{"recordings", "mbid"},
+	}
+
+	for _, s := range alterStmts {
+		stmt := fmt.Sprintf(
+			"ALTER TABLE %s ADD COLUMN %s TEXT", s.table, s.column,
+		)
+
+		if _, err := db.ExecContext(ctx, stmt); err != nil {
+			// Column may already exist from a partial migration.
+			if !strings.Contains(err.Error(), "duplicate column") {
+				return fmt.Errorf("migration 13: alter %s: %w", s.table, err)
+			}
+		}
+	}
+
+	// Partial indexes for MBID lookups (only index non-NULL rows).
+	indexes := []string{
+		"CREATE INDEX IF NOT EXISTS idx_artists_mbid ON artists(mbid) WHERE mbid IS NOT NULL",
+		"CREATE INDEX IF NOT EXISTS idx_release_groups_mbid ON release_groups(mbid) WHERE mbid IS NOT NULL",
+		"CREATE INDEX IF NOT EXISTS idx_recordings_mbid ON recordings(mbid) WHERE mbid IS NOT NULL",
+	}
+
+	for _, stmt := range indexes {
+		if _, err := db.ExecContext(ctx, stmt); err != nil {
+			return fmt.Errorf("migration 13: create index: %w", err)
+		}
+	}
+
+	if _, err := db.ExecContext(
+		ctx, "PRAGMA user_version = 13",
+	); err != nil {
+		return fmt.Errorf("could not set user_version to 13: %w", err)
+	}
+
+	logger.Info("migration 13 complete")
+
+	return nil
+}
+
 // readLibraryDirFromTOML reads the TOML config file and returns
 // the Library.DirectoryPath value, or "" if not configured.
 func readLibraryDirFromTOML(logger *slog.Logger) string {