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/core/formatting.py

@@ -345,6 +345,7 @@ METRICS: dict[str, tuple[str, str]] = {
     "updates_pending": ("Updates waiting", "packages the host's package manager would upgrade, checked hourly"),
     "updates_security": ("Security updates waiting", "of the updates waiting, those from a security source"),
     "reboot_required": ("Reboot required", "1 when the host needs a reboot to finish an update, 0 when not"),
+    "maintenance_failed": ("Maintenance failed", "1 when the host's latest scheduled or manual update failed, timed out or didn't come back into service"),
 }
 
 # Where a rule's metric comes from, as the picker and the rules list say it.