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.

packages/cloud-platform/cloud/scheduler.py

@@ -22,7 +22,9 @@ if TYPE_CHECKING:
     from cloud.store import CloudStore
 
 
-ScheduledTaskStatus = Literal["queued", "assigned", "dispatched", "done", "failed"]
+ScheduledTaskStatus = Literal[
+    "queued", "assigned", "dispatched", "done", "failed", "cancelled"
+]
 
 
 @dataclass(frozen=True)
@@ -57,6 +59,7 @@ class ScheduledTask:
     progress_step_status: str | None = None
     progress_summary: str | None = None
     progress_updated_at: datetime | None = None
+    cancel_requested_at: datetime | None = None
 
 
 @runtime_checkable