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/traefik/templates/traefik/rows.html

@@ -37,7 +37,7 @@
 </div>
 {% endmacro %}
 
-{% if problems or healthy or certs %}
+{% if problems or healthy or gone or certs %}
 {# ── Routers that need attention ─────────────────────────────────────────── #}
 {% if problems %}
 <div style="column-width:340px;column-gap:1rem;">
@@ -67,6 +67,22 @@
   </table>
   </div>
 </details>
+{% endif %}
+
+{# ── Routers Traefik no longer reports: their last numbers are history, not a
+   state, so they are listed by when they were last seen and nothing else. ── #}
+{% if gone %}
+<details class="card" style="margin-bottom:1rem;">
+  <summary class="section-title" style="cursor:pointer;">{{ gone|length }} router{{ 's' if gone|length != 1 }} no longer reported</summary>
+  <p class="field-hint" style="margin:0.5rem 0;">Traefik stopped reporting these, usually because the service behind them was removed.</p>
+  <ul class="problem-list">
+    {% for r in gone %}
+    <li><span style="word-break:break-all;">{{ r.name }}</span> <span class="problem-detail">last seen {{ r.latest.scraped_at|ago }}</span></li>
+    {% endfor %}
+  </ul>
+</details>
+{% endif %}
+{% if healthy %}
 <script>
   // The fragment is re-polled; keep the healthy list as the reader left it.
   (function () {