fix: auto-lookup album metadata from iTunes when missing on track download

Songs from new releases (Apple RSS feed) have no collectionName, causing
the album tag to be omitted from embedded metadata -> Navidrome shows
[Unknown Album]. Backend now calls iTunes search to fill in album, year,
trackNum, totalTracks, discNum, genre before downloading.

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

backend/novasonic-server.js

@@ -130,6 +130,35 @@ async function fetchLyrics(artist, title, album) {
   } catch { return null; }
 }
 
+// iTunes track lookup — fills in album, year, trackNum, totalTracks, genre when missing
+async function lookupItunesTrackMeta(artist, title) {
+  try {
+    const url = `https://itunes.apple.com/search?term=${encodeURIComponent(artist + ' ' + title)}&media=music&entity=musicTrack&limit=5&country=us&explicit=Yes`;
+    const results = await new Promise((resolve, reject) => {
+      https.get(url, res => {
+        let d = ''; res.on('data', c => d += c);
+        res.on('end', () => { try { resolve(JSON.parse(d).results || []); } catch { reject(new Error('parse')); } });
+      }).on('error', reject);
+    });
+    const norm = s => s?.toLowerCase().trim() || '';
+    const match = results.find(t =>
+      norm(t.trackName) === norm(title) &&
+      norm(t.artistName).split(' ')[0] === norm(artist).split(' ')[0]
+    ) || results[0];
+    if (!match) return {};
+    return {
+      album:       match.collectionName || '',
+      year:        match.releaseDate ? new Date(match.releaseDate).getFullYear() : undefined,
+      trackNum:    match.trackNumber   || undefined,
+      totalTracks: match.trackCount    || undefined,
+      discNum:     match.discNumber    || undefined,
+      discTotal:   match.discCount     || undefined,
+      genre:       match.primaryGenreName || undefined,
+      artworkUrl:  (match.artworkUrl100 || '').replace('100x100bb', '1000x1000bb'),
+    };
+  } catch { return {}; }
+}
+
 // Download a single track: yt-dlp audio + ffmpeg clean metadata + iTunes cover embedded
 async function downloadTrack({ jobId, artist, albumArtist, title, album, year, trackNum, totalTracks, discNum, discTotal, compilation, genre, artworkUrl, explicit, outputDir }) {
   const mainArtist = albumArtist || artist;
@@ -380,7 +409,14 @@ app.post('/api/download', async (req, res) => {
     updateJob(jobId, { status: 'running', progress: 5, message: 'Gestartet…' });
     try {
       if (type === 'track') {
-        const result = await downloadTrack({ jobId, artist, title, album, artworkUrl, explicit: !!explicit });
+        let meta = { album, year: undefined, trackNum: undefined, totalTracks: undefined, discNum: undefined, discTotal: undefined, genre: undefined };
+        if (!album) {
+          log(jobId, '📋 Suche Album-Metadaten von iTunes…');
+          const extra = await lookupItunesTrackMeta(artist, title);
+          if (extra.album) log(jobId, `📋 Album: "${extra.album}" (${extra.year || '?'})`);
+          meta = { ...extra, artworkUrl: artworkUrl || extra.artworkUrl || '' };
+        }
+        const result = await downloadTrack({ jobId, artist, title, artworkUrl: artworkUrl || meta.artworkUrl || '', explicit: !!explicit, ...meta });
         if (result?.skipped) {
           updateJob(jobId, { status: 'done', progress: 100, message: 'Bereits vorhanden – übersprungen', skipped: true });
         } else {