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/hosts/list.html

@@ -1,4 +1,8 @@
 {% extends "base.html" %}
+{% from "_macros.html" import status_badge, status_dot %}
+{# Hosts, problems first (host_svc.host_rank): a down monitor, then an agent
+   gone quiet, then monitors not yet checked, then the healthy. #}
+{% macro pct(v) %}{% if v is none %}—{% else %}{{ '%.0f'|format(v) }}%{% endif %}{% endmacro %}
 {% block title %}Hosts — Steward{% endblock %}
 {% block content %}
 <div style="display:flex;align-items:center;justify-content:space-between;margin-bottom:1.5rem;">
@@ -38,13 +42,7 @@
                 <td style="text-align:center;">
                     {% if not hs %}
                         <span style="color:var(--text-dim);font-size:0.8rem;" title="No monitors">—</span>
-                    {% elif hs.state == 'up' %}
-                        <span class="dot dot-up" title="Up"></span>
-                    {% elif hs.state == 'down' %}
-                        <span class="dot dot-down" title="Down"></span>
-                    {% else %}
-                        <span class="dot dot-dim" title="Pending"></span>
-                    {% endif %}
+                    {% else %}{{ status_dot(hs.state) }}{% endif %}
                 </td>
                 <td style="text-align:center;font-size:0.9rem;">
                     {% if hs and hs.latency_ms is not none %}
@@ -66,14 +64,13 @@
                     {% endif %}
                 </td>
                 {% endfor %}
-                {% set am = agent_metrics.get(host.name) %}
+                {% set am = agent.get(host.name) %}
                 <td style="text-align:center;font-size:0.8rem;font-family:ui-monospace,monospace;">
                     {% if am %}
-                        <span style="color:var(--text-muted);" title="CPU / memory">
-                            {{ '%.0f'|format(am.get('cpu_pct', 0)) }}% / {{ '%.0f'|format(am.get('mem_used_pct', 0)) }}%
-                        </span>
+                        <span style="color:var(--text-muted);" title="CPU / memory, latest">{{ pct(am.cpu_pct) }} / {{ pct(am.mem_used_pct) }}</span>
+                        {% if not am.fresh %}<div>{{ status_badge('stale', title="No agent report since " ~ (am.last_at.strftime('%Y-%m-%d %H:%M UTC') if am.last_at else 'it started')) }} <span class="tone-dim" style="font-family:inherit;">{{ am.last_at|ago }}</span></div>{% endif %}
                     {% else %}
-                        <span style="color:var(--text-dim);">—</span>
+                        <span style="color:var(--text-dim);" title="No agent on this host">—</span>
                     {% endif %}
                 </td>
                 <td class="td-actions">