feat(subsonic): expose track musicbrainz id

Release-As: 0.16.3

server/ctrlsubsonic/spec/construct_by_folder.go

@@ -74,6 +74,7 @@ func NewTCTrackByFolder(t *db.Track, parent *db.Album) *TrackChild {
 		Bitrate:       t.Bitrate,
 		IsDir:         false,
 		Type:          "music",
+		MusicBrainzID: t.TagBrainzID,
 		CreatedAt:     t.CreatedAt,
 		AverageRating: formatRating(t.AverageRating),
 	}

server/ctrlsubsonic/spec/construct_by_tags.go

@@ -77,6 +77,7 @@ func NewTrackByTags(t *db.Track, album *db.Album) *TrackChild {
 		TrackNumber:        t.TagTrackNumber,
 		DiscNumber:         t.TagDiscNumber,
 		Type:               "music",
+		MusicBrainzID:      t.TagBrainzID,
 		Year:               album.TagYear,
 		AverageRating:      formatRating(t.AverageRating),
 		TranscodeMeta:      TranscodeMeta{},

server/ctrlsubsonic/spec/spec.go

@@ -200,6 +200,9 @@ type TrackChild struct {
 	DiscNumber  int         `xml:"discNumber,attr,omitempty"  json:"discNumber,omitempty"`
 	Type        string      `xml:"type,attr,omitempty"        json:"type,omitempty"`
 	Year        int         `xml:"year,attr,omitempty"        json:"year,omitempty"`
+
+	MusicBrainzID string `xml:"musicBrainzId,attr"        json:"musicBrainzId"`
+
 	// star / rating
 	Starred       *time.Time `xml:"starred,attr,omitempty"         json:"starred,omitempty"`
 	UserRating    int        `xml:"userRating,attr,omitempty"      json:"userRating,omitempty"`