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/novasonic-server.js

@@ -18,7 +18,7 @@ const CONFIG    = require('./lib/config');
 const { jobs, updateJob, log } = require('./lib/jobs');
 const { lookupItunesTrackMeta, fetchItunesTracks, searchItunesAlbumTracks } = require('./lib/metadata');
 const { downloadTrack, downloadAlbumTracks } = require('./lib/downloader');
-const { buildRomajiIndex, clearRomajiCache, isTokenizerReady } = require('./lib/romaji');
+const { buildRomajiIndex, clearRomajiCache, isTokenizerReady, fetchNdSongs } = require('./lib/romaji');
 
 // ─── INIT ─────────────────────────────────────────────────────────────────────
 const app = express();
@@ -92,6 +92,32 @@ app.delete('/api/romaji-index', (req, res) => {
   res.json({ ok: true });
 });
 
+// GET /api/genre-playcounts?ndUrl=...&user=...&pass=...
+// Returns { [genre]: totalPlayCount } sorted by plays desc, cached for 20 min.
+const _genrePlayCache = new Map();
+const GENRE_PLAY_TTL  = 20 * 60 * 1000;
+app.get('/api/genre-playcounts', async (req, res) => {
+  const { ndUrl, user, pass } = req.query;
+  if (!ndUrl || !user || !pass) return res.status(400).json({ error: 'missing params' });
+  const key    = `${ndUrl}|${user}`;
+  const cached = _genrePlayCache.get(key);
+  if (cached && Date.now() - cached.ts < GENRE_PLAY_TTL) return res.json(cached.data);
+  try {
+    const songs = await fetchNdSongs(ndUrl, user, pass);
+    const totals = {};
+    for (const s of songs) {
+      const g = s.genre;
+      if (!g || !(s.playCount > 0)) continue;
+      totals[g] = (totals[g] || 0) + s.playCount;
+    }
+    _genrePlayCache.set(key, { ts: Date.now(), data: totals });
+    res.json(totals);
+  } catch (e) {
+    console.error('genre-playcounts error:', e);
+    res.status(500).json({ error: e.message });
+  }
+});
+
 app.post('/api/spotify/track-artwork', async (req, res) => {
   const tracks = req.body.tracks || [];
   if (!tracks.length) return res.json({ results: [] });