feat(health): Steward records its own health and Settings → System shows it (#5467)

#5467 "Steward's own health: metrics, rules can watch them, a page that shows them".

The self-monitor tick now records, under source "steward", resource "process":
ingest_posts / ingest_failed / ingest_slowest_ms (the host-agent posts since the
last tick, tallied in steward/core/health.py), db_roundtrip_ms (a timed SELECT 1),
and unwatched_secs (the gap before this tick past watch.GAP_S, from Steward's own
last sample). The hourly cleanup records cleanup_secs. The open-file pair is still
recorded, and is now only skipped on its own where /proc is absent. All of these
are in CORE_METRICS, so an alert rule can watch them under "Steward itself".

Settings → System (/settings/system/, admin) shows a verdict first (failed posts,
a gap in watching, open files near the limit), then each measure now and over the
last day, with a sparkline scaled to its own peak.

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

steward/app.py

@@ -414,8 +414,14 @@ def _register_core_tasks(app: Quart) -> None:
         await run_due_monitors(app)
 
     async def run_cleanup():
+        import time
         from .core.cleanup import run_cleanup as _cleanup
+        from .core.self_monitor import record_cleanup_secs
+        started = time.monotonic()
         await _cleanup(app)
+        # How long it took, as Steward's own metric (#5467); a failed run
+        # raises past this and records nothing.
+        await record_cleanup_secs(app, time.monotonic() - started)
 
     async def run_alert_reconcile():
         from .alerts.service import reconcile_covering_rules