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/status.py

@@ -105,25 +105,33 @@ def sparkline_svg(values: list[float], width: int = 80, height: int = 20,
 
 async def _last_n_results(db, monitor_ids: list[str],
                           n: int = HEARTBEAT_COUNT) -> dict[str, list]:
-    """Return {monitor_id: [MonitorResult]} of the last n results, oldest-first."""
-    from steward.models.monitors import MonitorResult
+    """Return {monitor_id: [MonitorResult]} of the last n results, oldest-first.
+
+    One LATERAL probe per monitor walks ix_monitor_results_monitor_checked
+    backwards and stops after n rows. A row_number() over each monitor's
+    results read its whole history first, which held Status and the monitor
+    widgets at ~16 s (#5208).
+    """
+    from sqlalchemy import true
+    from sqlalchemy.orm import aliased
+    from steward.models.monitors import Monitor, MonitorResult
     if not monitor_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).where(
-        MonitorResult.monitor_id.in_(monitor_ids)
-    ).subquery()
-    res = await db.execute(
+    ids = select(Monitor.id.label("mid")).where(Monitor.id.in_(monitor_ids)).subquery("wanted")
+    recent = (
         select(MonitorResult)
-        .join(subq, MonitorResult.id == subq.c.id)
-        .where(subq.c.rn <= n)
-        .order_by(MonitorResult.monitor_id, MonitorResult.checked_at.asc())
+        .where(MonitorResult.monitor_id == ids.c.mid)
+        .order_by(MonitorResult.checked_at.desc())
+        .limit(n)
+        .lateral("recent")
     )
+    row = aliased(MonitorResult, recent)
+    res = await db.execute(select(row).select_from(ids).join(recent, true()))
     out: dict[str, list] = {mid: [] for mid in monitor_ids}
-    for row in res.scalars():
-        out[row.monitor_id].append(row)
+    for r in res.scalars():
+        out[r.monitor_id].append(r)
+    for rows in out.values():
+        rows.sort(key=lambda r: r.checked_at)
     return out