chore: remove squid.wtf (Qobuz) backend — yt-dlp only

Deletes backend/lib/squid.js; strips all references from downloader.js,
config.js, novasonic-server.js, docker-compose.yml, .env.example, README,
and CHANGELOG. Default DOWNLOAD_BACKEND is now ytdlp.

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

backend/novasonic-server.js

@@ -386,7 +386,6 @@ app.get('/api/health', (req, res) => {
     musicPath: CONFIG.musicPath,
     audioFormat: CONFIG.audioFmt,
     downloadBackend: CONFIG.downloadBackend,
-    squidQuality: CONFIG.squidQuality,
     activeJobs: activeDownloads,
     queueLength: downloadQueue.length,
     spotify: { configured: true },
@@ -1428,10 +1427,6 @@ app.listen(CONFIG.port, () => {
   console.log(`\n🎵 NovaSonic Backend running on port ${CONFIG.port}`);
   console.log(`   Music folder     : ${CONFIG.musicPath}`);
   console.log(`   Audio format     : ${CONFIG.audioFmt} (quality ${CONFIG.audioQuality})`);
-  console.log(`   Download backend : ${CONFIG.downloadBackend}`);
-  if (CONFIG.downloadBackend !== 'ytdlp') {
-    console.log(`   Squid quality    : ${CONFIG.squidQuality}`);
-  }
   console.log(`   yt-dlp           : ${ytdlpVersion}`);
   console.log('');
 });