Add a background job safelyWorkflow

The request handler below does slow work ({{slow_work}}) inline:

{{code}}

Refactor it to a background job using {{queue_tool}}:
1. The handler validates input, enqueues a job with an idempotency key, and returns 202 immediately.
2. The worker is safe to run twice for the same key.
3. Retries use exponential backoff with a max attempt count; failures land in a dead-letter list.
4. Log job id, attempt and duration; never log secrets or full payloads.
5. Show how the UI can poll or subscribe for the result.

Return the new handler, the worker and a short runbook for stuck jobs.