Check Async Tasks

Generic server-side check of all in_progress async tasks.

Reads each task's structured check recipe and executes it:

1. Call check.url with check.method (GET/POST with check.body)
2. If POST returns job_id, wait poll_delay_s then poll the result
3. If response has check.success_key → deliver to conversation, mark done
4. If generating/pending → skip
5. If error → deliver error, mark done

This endpoint is fully generic — it doesn't know what the check URLs do. The recipes are written by the job submission layer (media.py, etc). Called by heartbeat: curl -s -X POST "$API_BASE_URL/api/tasks/agent/$AGENT_ID/check-async" -H "Authorization: Bearer $API_TOKEN"