feat: romaji index for Japanese kanji song search

Backend:
- Add /api/romaji-index endpoint using kuromoji morphological analyzer
- Fetches all Navidrome songs, converts kanji → hiragana readings → romaji
- Only indexes songs with Japanese characters; caches for 20 min
- /api/romaji-index DELETE clears cache (force rebuild)

Frontend:
- fetchRomajiIndex() in api.js: lazy-fetches and caches the index
- doLocalSearch: when query is romaji (ASCII), queries romaji index in
  parallel with Navidrome search3 and merges+deduplicates results

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

backend/package.json

@@ -0,0 +1,7 @@
+{
+  "name": "novasonic-backend",
+  "private": true,
+  "dependencies": {
+    "kuromoji": "^0.1.2"
+  }
+}