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/templates/settings/_tabs.html

@@ -10,5 +10,6 @@
                  ("auth", "Authentication", "/settings/auth/"),
                  ("users", "Users", "/settings/users/"),
                  ("ansible", "Ansible", "/settings/ansible/"),
-                 ("plugins", "Plugins", "/settings/plugins/")], active, "Settings") }}
+                 ("plugins", "Plugins", "/settings/plugins/"),
+                 ("system", "System", "/settings/system/")], active, "Settings") }}
 {%- endmacro %}