6 tests pin the Deezer contract:
- track_id stays as STRING (Deezer GW API uses string IDs).
- username slot is the legacy `'deezer_dl'` (frontend depends on it).
- Auth gate at top of `download()` returns None BEFORE thread spawn.
- Defensive fallback: filename without `||` synthesizes display name.
- Thread is named `deezer-dl-<track_id>` for diagnostics.
- State dict has Deezer-specific `error` slot.
