feat(host_agent): a group's updates and reboots on a weekly cadence, one host at a time (#5768)

#5768 "A maintenance cadence: a group's updates and reboots, one host at a
time, on a schedule". A plan names an inventory group, a weekday and time
in a time zone, a window and a reboot policy (only when required by
default). At its slot a run takes the group's hosts in order and gives each
a #5767 maintenance job. The next host starts only when the last is back in
service: its agent is posting and, for a swarm node, the node is active and
every replicated service is at its replicas. A failure stops the run, and
no host is started once the window has passed. A host's latest failure
shows as host_agent/<host>/maintenance_failed, for an ordinary alert rule.
The page is Hosts → Maintenance.

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

plugins/host_agent/templates/panel.html

@@ -27,7 +27,7 @@
     {% if maint_job %}
     {% set running = maint_job.state not in ('done', 'failed', 'timed_out') %}
     <div class="{{ 'tone-warn' if running or maint_job.state != 'done' else '' }}" title="{{ maint_job.detail or '' }}">
-      Maintenance: {{ maint_summary }}
+      <a href="/plugins/host_agent/maintenance" style="color:inherit;">Maintenance</a>: {{ maint_summary }}
       <span style="color:var(--text-muted);">· {% if running %}since {{ maint_job.state_at|ago }}{% else %}{{ maint_job.finished_at|ago }}{% endif %}{% if maint_job.state in ('failed', 'timed_out') and maint_job.detail %} · {{ maint_job.detail|truncate(90) }}{% endif %}</span>
     </div>
     {% endif %}