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>

.env.example

@@ -11,17 +11,7 @@ AUDIO_FMT=mp3          # mp3 | flac | opus | best  (used by yt-dlp)
 AUDIO_QUALITY=0        # 0 = best quality           (used by yt-dlp)
 MAX_PARALLEL=2
 
-# Download backend: ytdlp | squid | auto
-#   ytdlp  - always use yt-dlp
-#   squid  - always use squid.wtf (Qobuz)
-#   auto   - try squid.wtf first, fall back to yt-dlp if not found (default)
-DOWNLOAD_BACKEND=auto
-
-# Quality for squid.wtf / Qobuz downloads
-#   mp3    - MP3 320 kbps
-#   flac   - FLAC 16-bit / 44.1 kHz
-#   hi-res - FLAC 24-bit / 96 kHz
-SQUID_QUALITY=mp3
+DOWNLOAD_BACKEND=ytdlp
 
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