feat(web): switch the Apple/Erkunden search to Deezer

Apple's iTunes /search returns 403 from server IPs, killing the Erkunden search. Deezer
is free, needs no key, and is actually better: it returns explicit versions, artist
images, track durations and 30s previews.

- Backend: /api/deezer/* proxy (CORS-safe); album download accepts a `tracks` array in
  the request body (Deezer albums send their tracks directly, pre-mapped).
- api.js: itunesSearch / itunesLookupAlbum / itunesLookupArtistSongs|Albums now hit
  Deezer and map results to the existing ItunesItem shape, so the UI + download flow are
  unchanged. Function names kept.
- downloads.js: album downloads fetch the Deezer album's tracks and send them in the body.

iTunes /lookup (by ID) and the charts RSS proxy stay. Known follow-ups: artist-type
download still calls iTunes /search server-side; the metadata-editor iTunes autofill;
and the Android client — all still on iTunes for now.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'ns-v48';
+const CACHE = 'ns-v49';
 const PRECACHE = ['/', 'index.html', 'manifest.json', 'icon.svg'];
 const SKIP_PATHS = ['/rest/stream', '/rest/download', '/config.js'];
 // Auth token params stripped from cache keys so artwork survives across sessions/token refreshes.