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>

steward/templates/hosts/list.html

@@ -17,6 +17,7 @@
         {% if session.user_role == 'admin' %}
         <a class="btn btn-ghost" href="/plugins/host_agent/fleet/">Agent fleet</a>
         {% endif %}
+        <a class="btn btn-ghost" href="/plugins/host_agent/maintenance">Maintenance</a>
         {% if can_add %}
         <button type="button" class="btn" onclick="document.getElementById('add-host').showModal()">Add host</button>
         {% endif %}