fix: correct artist names and prevent wrong song downloads for Japanese tracks

- spotify.js: stop overwriting trackName with iTunes English title —
  yt-dlp must search with original Japanese title to find the right song
  (was downloading English covers e.g. Ghost In A Flower instead of 幽霊花)
- metadata.js: add title-only fallback in lookupItunesTrackMeta so tracks
  are found even when kuromoji gives wrong artist romanization (e.g.
  Terui Jun Sei vs iTunes Yoshimasa Terui); also return itunesArtistName
- novasonic-server.js: use itunesArtistName from iTunes lookup as the
  stored artist when available (single-track and playlist download paths)

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

backend/novasonic-server.js

@@ -364,7 +364,8 @@ app.post('/api/download', async (req, res) => {
         } else {
           log(jobId, `📋 Album aus Spotify: "${meta.album}"`);
         }
-        const result = await downloadTrack({ jobId, artist, title, ...meta, sourceUrl: sourceUrl || sp.sourceUrl || undefined });
+        const finalArtist = meta.itunesArtistName || artist;
+        const result = await downloadTrack({ jobId, artist: finalArtist, title, ...meta, sourceUrl: sourceUrl || sp.sourceUrl || undefined });
         if (result?.skipped) {
           updateJob(jobId, { status: 'done', progress: 100, message: 'Bereits vorhanden – übersprungen', skipped: true });
         } else {
@@ -438,10 +439,11 @@ app.post('/api/download', async (req, res) => {
               log(jobId, `📋 iTunes: "${trackArtist} – ${trackTitle}"`);
               itunes = await lookupItunesTrackMeta(searchArtist, trackTitle);
             }
+            const resolvedArtist = itunes.itunesArtistName || trackArtist;
             return downloadTrack({
               jobId,
-              artist:      trackArtist,
-              albumArtist: trackArtist,
+              artist:      resolvedArtist,
+              albumArtist: resolvedArtist,
               title:       trackTitle,
               album:       itunes.album       || track.collectionName || track.album || '',
               year:        itunes.year        || (track.releaseDate ? new Date(track.releaseDate).getFullYear() : undefined) || track.year,