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/alerts/service.py

@@ -27,7 +27,8 @@ CORE_METRICS: dict[str, list[str]] = {
     "tcp":     ["is_up", "response_ms"],
     "dns":     ["is_up", "response_ms"],
     "http":    ["is_up", "response_ms"],
-    "steward": ["open_fds", "open_fds_pct"],
+    "steward": ["ingest_posts", "ingest_failed", "ingest_slowest_ms", "db_roundtrip_ms",
+                "unwatched_secs", "cleanup_secs", "open_fds", "open_fds_pct"],
 }