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>

tests/integration/test_metric_latest.py

@@ -54,7 +54,8 @@ def test_the_newest_sample_wins_and_a_late_backlog_does_not_move_it_back(app):
                     _sample(host, "cpu_pct", 20.0, now - timedelta(minutes=1)),
                     _sample(host, "mem_used_pct", 40.0, now - timedelta(minutes=1)),
                 ])
-            after_first = await _latest(s, host)
+            async with s.begin():
+                after_first = await _latest(s, host)
             async with s.begin():
                 # An agent catching up posts older samples after newer ones.
                 await record_metrics(s, [