perf(monitors): a monitor's latest results by a LATERAL probe, not row_number over its history (#5208)

_last_n_results (Status, the monitors list, host monitors), host_status
(hosts overview, list_hosts) and the report's down-monitor list each
ranked every result a monitor ever recorded to keep the newest few. Each
monitor now gets ORDER BY checked_at DESC LIMIT n in a LATERAL subquery,
which walks ix_monitor_results_monitor_checked backwards and stops.

Also fixes the latest-table test, which read between two begin() blocks
and so autobegan a transaction the second block then refused.

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

steward/hosts/service.py

@@ -93,21 +93,12 @@ async def host_status(db, host_ids: list[str]) -> dict[str, dict]:
     """
     if not host_ids:
         return {}
-    rn = func.row_number().over(
-        partition_by=MonitorResult.monitor_id, order_by=MonitorResult.checked_at.desc()
-    ).label("rn")
-    subq = (
-        select(MonitorResult.id, rn)
-        .join(Monitor, Monitor.id == MonitorResult.monitor_id)
+    monitors = (await db.execute(
+        select(Monitor.id, Monitor.host_id)
         .where(Monitor.host_id.in_(host_ids), Monitor.enabled.is_(True))
-        .subquery()
-    )
-    rows = (await db.execute(
-        select(MonitorResult, Monitor.host_id)
-        .join(subq, MonitorResult.id == subq.c.id)
-        .join(Monitor, Monitor.id == MonitorResult.monitor_id)
-        .where(subq.c.rn == 1)
     )).all()
+    latest = await _last_n_results(db, [m.id for m in monitors], n=1)
+    rows = [(latest[mid][-1], host_id) for mid, host_id in monitors if latest.get(mid)]
     out: dict[str, dict] = {}
     for res, host_id in rows:
         d = out.setdefault(host_id, {"state": "pending", "latency_ms": None, "count": 0})