Get Batch Export Status
Polls the status of an asynchronous batch export job initiated by `POST /batches/{batch_id}/export`.
Returns the current status of the export:
- **generating** — the export is still being built; continue polling.
- **complete** — the export is ready; a presigned HTTPS download URL is included. URLs are valid for 1 hour and are automatically refreshed if expired.
- **failed** — generation failed (e.g. the archive was deleted from storage). Re-send `POST /batches/{batch_id}/export` to retry.