The Deezer write half of "export a mirrored playlist back to Deezer". Rides the private
gw-light gateway with the ARL session already authenticated for downloads (Deezer shut their
public developer API, so this is the only write path — unofficial and fragile by nature).
DeezerDownloadClient.create_or_update_playlist(name, track_ids, existing_id=None): playlist.create
with the songs (new), or playlist.addSongs to the stored target (re-export). track_ids are the
stored deezer_id per library track. Returns the same {success, playlist_id, url, added, error}
shape as the Spotify writer so the export job can treat both uniformly.
5 tests (mocked gateway): create sends playlist.create with positional songs, update sends
playlist.addSongs (no create), empty → error with no gw call, not-authed → error, gw rejection →
error. ruff clean. Additive — download paths untouched.
Both write clients done. Next: the export-job branch + endpoint (reusing get/set_playlist_export_
target for idempotency), then the modal options.
