Put work on a queue

This takes <n> seconds during a request and should be in the
background: <paste>

Stack: <stack>. I'd rather not run extra infrastructure unless needed.

- Recommend the simplest queue that fits, and say what it costs to
  operate.
- Enqueue returns immediately with an ID the user can poll or be
  notified against.
- The job is safe to run twice. Say how you made it so.
- Retries with backoff, a maximum attempt count, and a dead letter
  destination for jobs that never succeed.
- I must be able to see: pending, running, failed, and why.

Show me what the user sees at each stage, including failure.