feat(subsonic): add support for multi-valued album artist tags

closes #103

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scrobble/lastfm/scrobbler.go

@@ -29,6 +29,10 @@ func (s *Scrobbler) Scrobble(user *db.User, track *db.Track, stamp time.Time, su
 	if user.LastFMSession == "" {
 		return nil
 	}
+	if track.Album == nil || len(track.Album.Artists) == 0 {
+		return fmt.Errorf("track has no album artists")
+	}
+
 	apiKey, err := s.db.GetSetting("lastfm_api_key")
 	if err != nil {
 		return fmt.Errorf("get api key: %w", err)
@@ -46,13 +50,14 @@ func (s *Scrobbler) Scrobble(user *db.User, track *db.Track, stamp time.Time, su
 	} else {
 		params.Add("method", "track.updateNowPlaying")
 	}
+
 	params.Add("api_key", apiKey)
 	params.Add("sk", user.LastFMSession)
 	params.Add("artist", track.TagTrackArtist)
 	params.Add("track", track.TagTitle)
 	params.Add("trackNumber", strconv.Itoa(track.TagTrackNumber))
 	params.Add("album", track.Album.TagTitle)
-	params.Add("albumArtist", track.Artist.Name)
+	params.Add("albumArtist", track.Album.ArtistsString())
 	params.Add("duration", strconv.Itoa(track.Length))
 
 	// make sure we provide a valid uuid, since some users may have an incorrect mbid in their tags