sched: crash-safe admin ordering, create watermark, poll freshness (T13 fix)

tests/wf_client/test_schedules.py

@@ -128,6 +128,8 @@ async def test_schedule_snapshot_exposes_minimal_surface() -> None:
     assert schedule.enabled is True
     assert schedule.paused is False
     assert schedule.deleted is False
+    assert schedule.exhausted is False
+    assert schedule.blocked_reason is None
     assert schedule.overlap == "skip"
     assert schedule.misfire == "skip"
     assert schedule.max_active_runs == 1