feat: sort genre pills by total play count via backend

Page loads instantly with song-count order, then silently re-sorts
once the backend aggregates play counts across all songs. Result cached
20 min server-side to avoid repeated full-library fetches.

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

backend/lib/romaji.js

@@ -123,4 +123,4 @@ function clearRomajiCache() { romajiCache.clear(); }
 
 function isTokenizerReady() { return !!_tokenizer; }
 
-module.exports = { buildRomajiIndex, clearRomajiCache, isTokenizerReady };
+module.exports = { buildRomajiIndex, clearRomajiCache, isTokenizerReady, fetchNdSongs: _fetchNdSongs };