feat(cloud): add durable cancellation support to task repository

- 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.

tests/test_cloud_task_progress_visibility.py

@@ -90,7 +90,8 @@ def test_renew_lease_writes_progress_on_success(tmp_path) -> None:
             now=now + timedelta(seconds=5),
             progress=progress,
         )
-        assert result == "renewed"
+        assert result.status == "renewed"
+        assert result.cancel_requested is False
 
         task = database.repository.get_task(task_id)
         assert task is not None