feat: romaji artist names, Apple Music autofill, romaji index invalidation

- Artist grid and detail header now show romaji names when toggle is on.
  Builds a Map<artistId, artistRomaji> from the existing song romaji index
  so no extra API call is needed.

- Apple Music autofill button added to edit modal next to MusicBrainz.
  Reuses the existing lookupItunesTrackMeta backend function via a new
  GET /api/itunes/lookup endpoint — fills album, year, genre, track#,
  and cover art.

- Romaji index is invalidated after every successful download so newly
  added songs appear with correct sort tags without requiring a page reload.

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

backend/novasonic-server.js

@@ -570,6 +570,29 @@ app.post('/api/soundcloud/resolve', async (req, res) => {
   }
 });
 
+// ─── ITUNES LOOKUP ───────────────────────────────────────────────────────────
+app.get('/api/itunes/lookup', async (req, res) => {
+  const { artist, title } = req.query;
+  if (!artist || !title) return res.status(400).json({ error: 'artist and title required' });
+  try {
+    const meta = await lookupItunesTrackMeta(artist, title);
+    if (!meta.album && !meta.genre && !meta.year) return res.json({ found: false });
+    res.json({
+      found:       true,
+      album:       meta.album       || undefined,
+      year:        meta.year        || undefined,
+      trackNum:    meta.trackNum    || undefined,
+      totalTracks: meta.totalTracks || undefined,
+      genre:       meta.genre       || undefined,
+      coverUrl:    meta.artworkUrl  || undefined,
+      artist:      meta.itunesArtistName || undefined,
+    });
+  } catch (e) {
+    console.error('[itunes/lookup]', e.message);
+    res.status(500).json({ error: e.message });
+  }
+});
+
 // ─── MUSICBRAINZ LOOKUP ──────────────────────────────────────────────────────
 app.get('/api/musicbrainz/lookup', async (req, res) => {
   const { artist, title } = req.query;