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>

plugins/host_agent/routes.py

@@ -17,6 +17,7 @@ from sqlalchemy import select, or_, and_
 from datetime import timedelta
 
 from steward.core.formatting import worst_reading
+from steward.core.health import ingest as ingest_tally
 from steward.core.metric_series import points as metric_points
 from steward.core.settings import get_setting, public_base_url
 from steward.core.time_range import parse_range, RANGE_OPTIONS
@@ -368,7 +369,14 @@ async def ingest():
         # The agent hung up (its 10 s timeout) and the server dropped the
         # request: say how far it got before re-raising.
         phases.cancelled(token_hash[:8])
+        ingest_tally.done(phases.elapsed_ms(), ok=False)
         raise
+    except Exception:
+        ingest_tally.done(phases.elapsed_ms(), ok=False)
+        raise
+    # Counted for Steward's own health (#5467). The early returns above (a
+    # bad token, no valid samples) answer the agent and aren't counted.
+    ingest_tally.done(phases.elapsed_ms())
 
     phases.report(host.name, accepted, len(metric_rows),
                   sum(len(c) for _, c in docker_snapshots),
@@ -410,6 +418,9 @@ class _Phases:
         self.took: dict[str, float] = {}
         self.size = ""
 
+    def elapsed_ms(self) -> float:
+        return (time.monotonic() - self.start) * 1000.0
+
     def mark(self, name: str) -> None:
         now = time.monotonic()
         self.took[name] = self.took.get(name, 0.0) + (now - self.last)