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/core/reports.py

@@ -102,28 +102,13 @@ async def build_report_data(app) -> dict:
         ]
 
         # ── Monitors currently down (latest result is_up = false) ─────────────
-        latest_subq = (
-            select(MonitorResult.monitor_id, func.max(MonitorResult.checked_at).label("max_at"))
-            .group_by(MonitorResult.monitor_id)
-            .subquery()
-        )
-        down_result = await db.execute(
-            select(Monitor.name)
-            .join(MonitorResult, MonitorResult.monitor_id == Monitor.id)
-            .join(
-                latest_subq,
-                and_(
-                    MonitorResult.monitor_id == latest_subq.c.monitor_id,
-                    MonitorResult.checked_at == latest_subq.c.max_at,
-                ),
-            )
-            .where(
-                Monitor.enabled.is_(True),
-                MonitorResult.is_up.is_(False),
-            )
-            .order_by(Monitor.name)
-        )
-        hosts_down = [row.name for row in down_result]
+        from steward.core.status import _last_n_results
+        enabled = (await db.execute(
+            select(Monitor.id, Monitor.name).where(Monitor.enabled.is_(True))
+        )).all()
+        latest = await _last_n_results(db, [m.id for m in enabled], n=1)
+        hosts_down = sorted(name for mid, name in enabled
+                            if latest.get(mid) and not latest[mid][-1].is_up)
 
     return {
         "generated_at": now,