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>

tests/core/test_host_page.py

@@ -304,7 +304,7 @@ async def test_the_agent_card_follows_a_maintenance_job_and_says_why_it_cant_sta
             maint_blocker="a maintenance job is already running on it", **kw)
         free = await render_plugin_template(app, "host_agent", "panel.html", maint_job=None,
                                             maint_blocker=None, **kw)
-    assert busy.index("Maintenance: Rebooting") < busy.index("4 updates waiting")
+    assert busy.index("Maintenance</a>: Rebooting") < busy.index("4 updates waiting")
     assert "Can't update from here: a maintenance job is already running on it." in busy
     assert ">Update now</button>" not in busy
     assert ">Update now</button>" in free and 'name="reboot_policy" value="never"' in free