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/settings/routes.py

@@ -623,6 +623,22 @@ async def _reports_page(settings: dict, flash):
                                  shown_day=shown_day, shown_hour=shown_hour, last_sent=last_sent)
 
 
+@settings_bp.get("/system/")
+@require_role(UserRole.admin)
+async def system():
+    """Steward's own health (#5467): its process metrics now and over the day,
+    each one an alert rule can watch (source "Steward itself")."""
+    from steward.core.health import process_health
+    from steward.core.status import sparkline_svg
+    async with current_app.db_sessionmaker() as db:
+        health = await process_health(db)
+    for row in health["rows"]:
+        # From zero to the row's own peak, with a floor (UI & Design ruling 8).
+        row["spark"] = sparkline_svg(row["series"], width=120, height=20,
+                                     stroke="currentColor", floor=1.0) if len(row["series"]) > 1 else ""
+    return await render_template("settings/system.html", **health)
+
+
 @settings_bp.get("/reports/")
 @require_role(UserRole.admin)
 async def reports():