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>

CHANGELOG.md

@@ -62,6 +62,9 @@ Versioning: `MAJOR.MINOR.PATCH` — minor = new feature, patch = fix/improvement
 - **Android** — Artist fast-scroll index bar: draggable letter strip on the right side of the Artists list with letter bubble while dragging
 - **Web** — Playlist edit modal: "Cover entfernen" button removed
 
+### Removed
+- **Backend** — squid.wtf (Qobuz) download backend removed; downloads now exclusively use yt-dlp. `DOWNLOAD_BACKEND` and `SQUID_QUALITY` env vars are no longer read.
+
 ### Fixed
 - **Android** — In-app notification bell: notifications were never delivered after navigating away from the Importer screen (poll ran in `viewModelScope`, cancelled on navigation); polling now uses `NotificationStore.startPoll()` with an app-lifetime scope
 - **Android** — Home hero card: pool now matches web — union of `frequent(5) ∪ recentlyPlayed(5)` deduped, random pick; label is "MEISTGESPIELT" or "ZULETZT GEHÖRT" (was hardcoded "AKTUELL")