feat: monitors, UniFi and Traefik keep collecting through a storage stall (#5995)

The same Holding as the SNMP poll, for the rest of Steward's own
collectors:

- Monitors: the list is read within 10 s or the last one read stands in;
  a start the database doesn't take is kept in memory, so a monitor isn't
  checked twice; each result is held and stored at the time it was
  checked. A stall shows on the monitors as the checks it saw, not a gap.
- UniFi and Traefik: each poll's writes are held (hold_write) and run
  oldest first in one transaction; every reading is stamped with its scrape.
- A held round the database refuses (as opposed to can't be reached) is
  stored alone and let go, so it can't hold up every round after it.

The Prometheus scrape is left as it is: its one target, Fabled Archives,
keeps its own database on the same NFS.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core/test_monitor_schedule.py

@@ -152,3 +152,41 @@ async def test_nothing_due_stamps_nothing(monkeypatch):
     app = _app(store)
     await sched.run_due_monitors(app)
     assert store.stamped == [] and store.added == []
+
+
+class _HungSession(_Session):
+    """The storage under Postgres has stalled: nothing answers until `gate`."""
+
+    def __init__(self, store, gate):
+        super().__init__(store)
+        self.gate = gate
+
+    async def __aenter__(self):
+        await self.gate.wait()
+        return self
+
+
+async def test_checks_go_on_while_the_database_hangs_and_are_stored_at_their_own_time(monkeypatch):
+    async def fake_run(m):
+        return dict(sched._BLANK, is_up=False, error_msg="502 Bad Gateway")
+
+    monkeypatch.setattr(sched, "run_monitor", fake_run)
+    monkeypatch.setattr(sched, "MetricBatch", _Batch)
+    monkeypatch.setattr(sched, "MONITORS_READ_S", 0.05)
+    monkeypatch.setattr(sched, "STORE_WAIT_S", 0.05)
+    store, gate = _Store([_m("app")]), asyncio.Event()
+    app = NS(config={"MONITORS_POLL_INTERVAL": 60}, db_sessionmaker=lambda: _HungSession(store, gate))
+    sched._memory(app).monitors = list(store.monitors)        # as last read, before the stall
+
+    await sched.run_due_monitors(app)                          # the list and the stamp time out
+    await _settle(app)                                         # the result is held, the monitor freed
+    assert store.added == [] and sched._state(app)[0] == set()
+    await sched.run_due_monitors(app)
+    await _settle(app)
+    assert store.added == []                                   # its start was kept in memory: not due again
+
+    checked_by = datetime.now(timezone.utc)
+    gate.set()                                                 # the database answers
+    await app._holdings["Monitor checks"].flush(1.0)
+    [res] = store.added
+    assert res.is_up is False and res.checked_at < checked_by  # stored at the time it was checked