Closed-book card for day 63. This file is the interviewer. It is not a worked example.
Do not open the Phase 4 lessons while this timer is running. Do not scroll past the attempt barrier on the day 63 page. This is not product search, video, nearby, ticket booking, or metrics ingestion.
Design a job scheduler.
Users define jobs that must run on a schedule. The system fires each run near the planned time, retries failures, and must not leave a job stuck if a worker dies mid-run. The same scheduled fire must not do the side effect twice if the worker retries.
You have 35 minutes. Blank page. No notes.