fix(spotify): iTunes lookup per track in playlist downloads

Spotify's flat embed format provides no album, year or per-track artwork.
Now calls lookupItunesTrackMeta for every playlist track before downloading:
- Uses first artist of "Artist1, Artist2" subtitle for iTunes search query
  (better match), keeps full string as the MP3 tag
- iTunes album/year/trackNum/artwork override the empty Spotify fields
- Falls back to Spotify data if iTunes returns nothing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/novasonic-server.js

@@ -648,16 +648,26 @@ app.post('/api/download', async (req, res) => {
         let done = 0, skipped = 0;
         for (const track of plTracks) {
           try {
+            const trackTitle  = track.trackName  || '';
+            // subtitle gives "Artist1, Artist2" — use first for iTunes search, keep full for tags
+            const trackArtist = track.artistName || artist || '';
+            const searchArtist = trackArtist.split(',')[0].trim();
+
+            // Always look up iTunes for correct album, artwork, year (Spotify flat format has none)
+            log(jobId, `📋 iTunes: "${trackArtist} – ${trackTitle}"`);
+            const itunes = await lookupItunesTrackMeta(searchArtist, trackTitle);
+
             const result = await downloadTrack({
               jobId,
-              artist:      track.artistName    || artist || '',
-              albumArtist: track.artistName    || artist || '',
-              title:       track.trackName     || '',
-              album:       track.collectionName || '',
-              year:        track.releaseDate ? new Date(track.releaseDate).getFullYear() : undefined,
-              trackNum:    track.trackNumber   || undefined,
-              genre:       track.primaryGenreName || undefined,
-              artworkUrl:  track.artworkUrl100 || artworkUrl || '',
+              artist:      trackArtist,
+              albumArtist: trackArtist,
+              title:       trackTitle,
+              album:       itunes.album      || track.collectionName || '',
+              year:        itunes.year       || (track.releaseDate ? new Date(track.releaseDate).getFullYear() : undefined),
+              trackNum:    itunes.trackNum   || track.trackNumber   || undefined,
+              totalTracks: itunes.totalTracks || undefined,
+              genre:       itunes.genre      || track.primaryGenreName || undefined,
+              artworkUrl:  itunes.artworkUrl || track.artworkUrl100 || '',
               explicit:    track.trackExplicitness === 'explicit',
             });
             if (result?.skipped) skipped++; else done++;