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/audit/service.py

@@ -138,6 +138,10 @@ ACTIONS = {
     "host.created": "added host",
     "host.renamed": "renamed host",
     "host_agent.maintenance": "started maintenance (update, reboot if needed) on host",
+    "host_agent.maintenance_plan_created": "added maintenance plan",
+    "host_agent.maintenance_plan_toggled": "paused or resumed maintenance plan",
+    "host_agent.maintenance_plan_deleted": "deleted maintenance plan",
+    "host_agent.maintenance_plan_run": "ran maintenance plan now",
     "host.deleted": "deleted host",
     "monitor.created": "added monitor",
     "monitor.updated": "changed monitor",