feat: format multi-artist credits

internal/app/app.go

@@ -1358,6 +1358,7 @@ func buildTagMetadata(trackMeta map[string]any, title, source, trackID string, o
 	if artist == "" {
 		artist = jsonutil.NestedString(trackMeta, "artist", "name")
 	}
+	artistNames := stringSliceFromAny(trackMeta["artist_names"])
 	albumArtist := jsonutil.NestedString(trackMeta, "album", "artist", "name")
 	if albumArtist == "" {
 		albumArtist = artist
@@ -1469,6 +1470,7 @@ func buildTagMetadata(trackMeta map[string]any, title, source, trackID string, o
 		Title:               title,
 		Album:               album,
 		Artist:              artist,
+		Artists:             artistNames,
 		AlbumArtist:         albumArtist,
 		OmitDiscTags:        opts.forPlaylist,
 		TrackNumber:         trackNumber,
@@ -1494,6 +1496,24 @@ func buildTagMetadata(trackMeta map[string]any, title, source, trackID string, o
 	}
 }
 
+func stringSliceFromAny(v any) []string {
+	items, ok := v.([]string)
+	if ok {
+		return append([]string(nil), items...)
+	}
+	rawItems, ok := v.([]any)
+	if !ok {
+		return nil
+	}
+	out := make([]string, 0, len(rawItems))
+	for _, raw := range rawItems {
+		if s := strings.TrimSpace(jsonutil.StringFromAny(raw)); s != "" {
+			out = append(out, s)
+		}
+	}
+	return out
+}
+
 func normalizeInitialKey(in string) string {
 	s := strings.TrimSpace(in)
 	if s == "" {