Same migration pattern as C2-C5. Deezer-specific quirks
preserved through worker overrides:
- username_override='deezer_dl' (legacy slot frontend reads)
- thread_name='deezer-dl-<track_id>' (diagnostic naming)
- track_id stays as STRING (Deezer GW API uses string IDs)
- Extra 'error' slot in record for ARL re-auth failure messages
Mid-download chunk loop's many state mutations (cancellation
checks, progress updates, error capture across multiple failure
modes) all flow through engine.update_record / get_record now.
Added _set_error and _is_cancelled helpers to keep call sites
readable.
Pinning tests updated. Suite still green (319 download tests).
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.
