feat(host_agent): Steward hands a host a maintenance job and follows it through the reboot (#5767)

#5767 "Steward hands a host a maintenance job and follows it to the end,
through its own restarts", milestone 514.

A host_maintenance_jobs row per job: queued → updating → [draining →]
rebooting → [undraining →] done, or failed / timed_out. Steward's answer to
an ingest post carries the step that host's agent should carry out now; the
agent (1.18.0) leaves each step id for the root helper once, and the helper
refuses an id it already ran, so repeats are harmless. Results come back as
the #5766 maintenance events, matched by step id; the reboot completes when
the host's `started` event has a boot_at after the reboot was handed out.

The job reboots only when the update says one is needed (or never, if asked).
A swarm node is drained first by itself if it's a manager or by a manager
with the helper, and made active after; with no manager to drain it the job
is refused. A minute tick times out a job past its state's deadline. A
maintenance window over host_agent/<host> lasts the rest of the job.

Everything is on the row, so a Steward restart, or the reboot of the host
Steward runs on, resumes from the next post. Ingest runs it in its own
savepoint so a job going wrong never costs the samples.

The Agent panel leads with a running job and says what a finished one did;
Manage agent has Update now (with "never reboot"), or says why a job can't
start. Integration test: hand off, Steward restarts, the post-boot events
close the job.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/host_agent/__init__.py

@@ -37,8 +37,8 @@ def setup(app: "Quart") -> None:
 
 
 def get_scheduled_tasks() -> list:
-    from .scheduler import make_task
-    return [make_task(_app)]
+    from .scheduler import make_maintenance_task, make_task
+    return [make_task(_app), make_maintenance_task(_app)]
 
 
 def get_blueprint():