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/downloader.js

@@ -1,24 +1,9 @@
-const CONFIG       = require('./config');
 const { log, updateJob } = require('./jobs');
-const ytdlp        = require('./ytdlp');
-const squid        = require('./squid');
+const ytdlp              = require('./ytdlp');
+const CONFIG             = require('./config');
 
-// Route a single track download to the correct backend, with optional fallback.
+// Download a single track via yt-dlp.
 async function downloadTrack(opts) {
-  const backend = CONFIG.downloadBackend;
-
-  if (backend === 'ytdlp') return ytdlp.downloadTrack(opts);
-  if (backend === 'squid') return squid.downloadTrack(opts);
-
-  // 'auto': squid first, yt-dlp as fallback
-  try {
-    const result = await squid.downloadTrack(opts);
-    if (result !== null) return result;
-    // null = not found on Qobuz — fall through to yt-dlp
-    log(opts.jobId, '⚠️  squid.wtf: nicht gefunden — Fallback zu yt-dlp');
-  } catch (e) {
-    log(opts.jobId, `⚠️  squid.wtf Fehler — Fallback zu yt-dlp: ${e.message}`);
-  }
   return ytdlp.downloadTrack(opts);
 }