- Add nullable cancel_requested_at column (migration 0012)
- Widen ScheduledTaskStatus/TerminalTaskStatus to include cancelled
- Add CancellationRequestStatus + request_task_cancellation() to
CloudRepository protocol and SQLAlchemy implementation
- renew_lease() now returns LeaseRenewalResult, surfacing whether
cancellation is pending, instead of a bare status string
- reap_expired_leases() resolves pending-cancellation tasks to
cancelled instead of requeuing/failing them
- record_task_result() accepts cancelled and clears
cancel_requested_at on any terminal write
Note: internal_api/api.py's renew_assignment route still compares
renew_lease()'s return value against a bare string; it will be
updated in the next task (Internal Host<->Cloud protocol) to consume
LeaseRenewalResult and populate the new cancel_requested wire field.