feat(alerts): one evaluation path; host-agent ingest fires rules (#5060)

#5060 "One evaluation path: record_metrics batch API; host-agent ingest
evaluates rules".

- record_metrics(session, rows) writes metric rows and evaluates the rules
  that watch them, reading rules, states and maintenance windows once per
  batch. Each rule sees its key's samples oldest first, so an agent catching
  up after an outage counts consecutive checks as they happened.
- Host-agent ingest now goes through it. Before, it wrote plugin_metrics
  directly and no host_agent rule could ever fire. The catalog gains the
  per-mount disk_used_pct ("<host>:<mount>").
- MetricBatch gathers a transaction's samples; the docker ingest, traefik,
  unifi, snmp, monitor and self-monitor writers record one batch each instead
  of a rule query per sample.

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

steward/monitors/scheduler.py

@@ -6,7 +6,7 @@ from datetime import datetime, timezone
 
 from sqlalchemy import select
 
-from steward.core.alerts import record_metric
+from steward.core.alerts import MetricBatch
 from steward.models.monitors import Monitor, MonitorResult
 from .runner import run_monitor
 
@@ -63,14 +63,16 @@ async def run_due_monitors(app) -> None:
                     db_monitor.last_checked_at = check_time
 
                 # Alert pipeline — keyed by monitor type (source) + name.
-                await record_metric(
-                    session=session, source_module=monitor.type,
+                batch = MetricBatch()
+                batch.add(
+                    source_module=monitor.type,
                     resource_name=monitor.name, metric_name="is_up",
                     value=1.0 if res["is_up"] else 0.0,
                 )
                 if res["response_ms"] is not None:
-                    await record_metric(
-                        session=session, source_module=monitor.type,
+                    batch.add(
+                        source_module=monitor.type,
                         resource_name=monitor.name, metric_name="response_ms",
                         value=res["response_ms"],
                     )
+                await batch.record(session)