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>

docker-compose.yml

@@ -30,8 +30,7 @@ services:
       - AUDIO_FMT=${AUDIO_FMT:-mp3}
       - AUDIO_QUALITY=${AUDIO_QUALITY:-0}
       - MAX_PARALLEL=${MAX_PARALLEL:-2}
-      - DOWNLOAD_BACKEND=${DOWNLOAD_BACKEND:-auto}
-      - SQUID_QUALITY=${SQUID_QUALITY:-mp3}
+      - DOWNLOAD_BACKEND=${DOWNLOAD_BACKEND:-ytdlp}
       - TICKETMASTER_API_KEY=${TICKETMASTER_API_KEY}
     command: >
       sh -c "