feat(ui): host page leads with its alerts; old readings say how old (#5210)

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

- Host page: an "Alerts on this host" strip first (rules on its agent
  series, its containers or its monitors: alert_svc.watches_host); its
  monitors problems first with "Down 12m · reason"; Docker above Ansible.
- Uptime page and widget: worst 24h uptime first, then 7d and 30d; hosts
  with no uptime yet last.
- Agent panel and fleet widget: last seen as "3m ago", not a clock time.
- SNMP: a device is "Reachable" only while it answered within the last 3
  polls; older readings say "No answer" with their age, and every device
  says when it was last read (was: reachable whenever any reading existed).
- Traefik: a router missing from 3 scrapes in a row is listed apart as no
  longer reported, by when it was last seen, instead of keeping its last
  state (a removed router stayed Failing).

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

plugins/snmp/templates/snmp/index.html

@@ -12,9 +12,14 @@
 {% for device in devices %}
 <section class="card" aria-labelledby="snmp-{{ loop.index }}">
   <div class="host-health-head">
-    {{ status_badge('up', 'Reachable') if device.readings else status_badge('pending', 'No data yet') }}
+    {#- reach: answered within the last few polls (up), only long ago (stale),
+        or never (pending); routes._device_state. #}
+    {% if device.reach == 'up' %}{{ status_badge('up', 'Reachable') }}
+    {% elif device.reach == 'stale' %}{{ status_badge('stale', 'No answer', title="The last reading is from " ~ device.at.strftime('%Y-%m-%d %H:%M UTC') ~ "; the values below are from then") }}
+    {% else %}{{ status_badge('pending', 'No data yet') }}{% endif %}
     <h2 id="snmp-{{ loop.index }}" class="section-title" style="margin:0;">{{ device.name }}</h2>
     <span class="field-hint mono">{{ device.host }}</span>
+    {% if device.at %}<span class="field-hint">read {{ device.at|ago }}</span>{% endif %}
     <a href="/plugins/snmp/device/{{ device.name }}" class="btn btn-sm btn-ghost" style="margin-left:auto;">History</a>
   </div>