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/backup/contents.py

@@ -16,6 +16,8 @@ from __future__ import annotations
 KEPT: frozenset[str] = frozenset({
     "app_settings", "users", "user_invites", "api_tokens",
     "hosts", "host_agent_registrations",
+    # A group's maintenance cadence (#5768): configuration.
+    "host_maintenance_plans",
     "monitors",
     "alert_rules", "alert_states", "maintenance_windows",
     "ansible_targets", "ansible_groups", "ansible_target_groups", "ansible_schedules",
@@ -39,7 +41,7 @@ EXCLUDED: frozenset[str] = frozenset({
     # A host's downtime as its agent reported it: history, like the metrics.
     "host_downtime",
     # Maintenance jobs (#5767): run history, like ansible_runs.
-    "host_maintenance_jobs",
+    "host_maintenance_jobs", "host_maintenance_runs",
     "email_outbox", "email_outbox_events", "password_resets",
     # Run history; its logs live on disk, not in the database.
     "ansible_runs",