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/templates/monitors/rows.html

@@ -22,6 +22,11 @@
       <a class="ping-name" href="/monitors/{{ m.id }}">{{ m.name }}</a>
     </div>
     <span class="ping-addr" title="{{ m.target }}">{{ m.type|monitor_type }} · {{ m.target }}</span>
+    {% if d.state == 'down' %}
+    <span class="tone-crit" style="font-size:0.75rem;overflow-wrap:anywhere;">
+      Down{% if d.down_since %} {{ d.down_since|ago|replace(' ago', '') }}{% endif %}{% if d.down_reason %} · {{ d.down_reason }}{% endif %}
+    </span>
+    {% endif %}
   </div>
 
   <div class="ping-pills">