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/lib/config.js

@@ -6,10 +6,7 @@ const CONFIG = {
   audioFmt:        process.env.AUDIO_FMT       || 'mp3',
   audioQuality:    process.env.AUDIO_QUALITY   || '0',
   maxParallel:     parseInt(process.env.MAX_PARALLEL || '2', 10),
-  // 'ytdlp' | 'squid' | 'auto' (squid first, yt-dlp fallback)
-  downloadBackend: (process.env.DOWNLOAD_BACKEND || 'auto').toLowerCase(),
-  // squid.wtf quality: 'mp3' | 'flac' | 'hi-res'
-  squidQuality:       (process.env.SQUID_QUALITY        || 'mp3').toLowerCase(),
+  downloadBackend: (process.env.DOWNLOAD_BACKEND || 'ytdlp').toLowerCase(),
   ticketmasterKey:     process.env.TICKETMASTER_API_KEY  || '',
 };