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/lib/metadata.js

@@ -103,21 +103,25 @@ async function lookupItunesTrackMeta(artist, title) {
     const normA     = (artist || '').toLowerCase().trim();
     // Split by space OR comma/semicolon/slash so "VXLLAIN, iGRES" → "vxllain" not "vxllain,"
     const firstWord = s => (s || '').toLowerCase().split(/[\s,;/]/)[0];
-    const match  = results.find(t =>
+    // Prefer strict match (title + first-word artist), fall back to title-only.
+    // Title-only fallback handles cases where kuromoji gave a wrong artist romanization
+    // (e.g. "Terui Jun Sei" vs iTunes "Yoshimasa Terui" — different first word).
+    const match = results.find(t =>
       normalizeTitle(t.trackName) === normT &&
       firstWord(t.artistName) === firstWord(normA)
-    );
+    ) || results.find(t => normalizeTitle(t.trackName) === normT);
     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'),
-      durationMs:  match.trackTimeMillis  || undefined,
+      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'),
+      durationMs:       match.trackTimeMillis  || undefined,
+      itunesArtistName: match.artistName       || undefined,
     };
   } catch { return {}; }
 }