feat(ui): alerts, monitors, hosts, schedules and maintenance lists put problems first, with since-times (#5210)

#5210 "Problems first, with since-time, across every list" (milestone 477),
first batch.

- Alerts: the Active list reads firing, no data, acknowledged, pending, the
  longest-running first, and re-polls itself. An acknowledged alert says how
  long it has been breaching (from the alert's events; acknowledging moves
  last_transition_at) and, apart from that, who acknowledged it and when.
  Rules show what each is doing now; disabled rules sort last. The current
  value reads the latest-value table (#5208) instead of plugin_metrics.
- Monitors: a down monitor says "Down 12m · connection refused" in the list.
- Hosts: sorted down, agent gone quiet, pending, healthy. The agent column
  shows "—" for a missing reading (was 0%) and a Stale badge with its age.
  agent_metrics_by_host is gone; the list reads agent_overview_by_host.
- Schedules: failing first (failed or interrupted run, or a firing that could
  not start one), then running, then the rest, paused last; status badge
  from the shared states.
- Maintenance windows: active (ending soonest), then upcoming (starting
  soonest), with expired folded away; each says when it ends/starts/ended.

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

steward/hosts/routes.py

@@ -50,14 +50,17 @@ async def list_hosts():
         hosts = result.scalars().all()
         host_status = await host_svc.host_status(db, [h.id for h in hosts])
         uptime = await host_svc.compute_uptime(db)
-        agent_metrics = await host_svc.agent_metrics_by_host(db, [h.name for h in hosts])
+        agent = await host_svc.agent_overview_by_host(
+            db, [h.name for h in hosts], host_svc.agent_stale_after_seconds(current_app.config))
 
+    hosts = sorted(hosts, key=lambda h: (
+        host_svc.host_rank(host_status.get(h.id), agent.get(h.name)), h.name.lower()))
     return await render_template(
         "hosts/list.html",
         hosts=hosts,
         host_status=host_status,
         uptime=uptime,
-        agent_metrics=agent_metrics,
+        agent=agent,
     )