fix: playlist matching for romaji-stored Japanese tracks + punctuation cleanup

- Clean Japanese punctuation (、。・) in romajiForStorage so it doesn't pass through
- Add POST /api/romaji-convert endpoint for batch title conversion
- Add romajiConvert() to api.js
- Add third fallback in _createNdPlaylist: convert Japanese title to romaji and
  search Navidrome with it (for songs already downloaded with romaji tags)

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

backend/novasonic-server.js

@@ -109,6 +109,14 @@ app.delete('/api/romaji-index', (req, res) => {
   res.json({ ok: true });
 });
 
+// POST /api/romaji-convert — convert an array of strings to romaji
+app.post('/api/romaji-convert', (req, res) => {
+  const { texts } = req.body;
+  if (!Array.isArray(texts)) return res.status(400).json({ error: 'texts must be array' });
+  if (!isTokenizerReady()) return res.status(503).json({ error: 'tokenizer not ready' });
+  res.json({ results: texts.map(t => romajiForStorage(t) || t) });
+});
+
 // GET /api/genre-playcounts?ndUrl=...&user=...&pass=...
 // Returns { [genre]: totalPlayCount } sorted by plays desc, cached for 20 min.
 const _genrePlayCache = new Map();