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/host_agent/templates/widget_table.html

@@ -39,7 +39,7 @@
         <span style="font-weight:500;font-size:0.85rem;overflow-wrap:anywhere;">{{ r.host.name }}</span>
         {% endif %}
         <span style="display:block;font-size:0.7rem;color:var(--text-dim);">
-          {{ r.reg.last_seen_at.strftime("%H:%M:%S") if r.reg.last_seen_at else "never" }}
+          {% if r.reg.last_seen_at %}seen {{ r.reg.last_seen_at|ago }}{% else %}never seen{% endif %}
         </span>
       </span>
     </div>