feat(maintenance): a run spreads swarm services back out after its last host, moving only what the emptier node can start (#6398)

Each weekly run drains Docker02 then Docker03, and the swarm never moves a
task back by itself, so every run ended with nearly every service on one
node. On 2026-10-25 the operator had that spread out by hand.

- Agent 1.26.0 reports each swarm service's pinned digest, and every
  repo@sha256 its host holds (from /system/df, every 10 min).
- Once a run's last host is back in service, Steward picks services on the
  fullest node to move to the emptiest until they are within one task. It
  takes services with several replicas on one node first. A service moves only
  when the emptier node already holds its exact image. One the plan keeps in
  place, one below its replicas, and one with no pinned digest stay put, and
  the run says why.
- A swarm manager's helper carries it out through a new action, rebalance,
  with the service names (validated, at most 48). It force-updates each in
  turn with the spec unchanged and no registry lookup, skips any a
  constraint places, and gives each 5 min to settle.
- The run ends done with what moved, or failed if a move didn't settle or
  the manager didn't answer in 60 min. A docker-scoped window covers the
  restarts, and the step goes on the timeline.
- Plans gain "Spread swarm services back out" (on by default) and "Keep in
  place", on the create form and in an "After the run" form on each plan.
  list_maintenance_plans shows both and the latest run's spread.

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

plugins/host_agent/cadence.py

@@ -10,7 +10,8 @@ active, every replicated service at its replicas. A job that fails, or a host
 that doesn't come back into service, stops the run, and the host's
 maintenance_failed figure goes to 1 for an alert rule to catch. No host is
 started after the window; the run then stops and says which hosts it didn't
-reach.
+reach. A run that drained swarm nodes ends by spreading the swarm's services
+back out (rebalance.py), unless its plan says not to.
 
 The decisions are model-free here (plans and runs are anything with the
 attributes); the database side is below them, and the plugin's minute tick
@@ -22,7 +23,7 @@ from __future__ import annotations
 import json
 from datetime import datetime, time, timedelta, timezone
 
-from . import maintenance, timeline
+from . import maintenance, rebalance, timeline
 
 WEEKDAYS = ("Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday")
 # How long a host may take to be back in service after its job finished.
@@ -244,11 +245,14 @@ async def advance(session, run, plan, now: datetime) -> str:
                 await timeline.write(session, f"maintenance job {job.id}",
                                      timeline.job_entry(session, job, host.name if host else None))
             name = host.name if host else host_id
-            _end(run, "failed", now, f"{name}: {why}")
+            _end(run, "failed", f"{name}: {why}", now=now)
         elif move == "stop":
-            _end(run, "stopped", now, why)
+            _end(run, "stopped", why, now=now)
         elif move == "done":
-            _end(run, "done", now, None)
+            if await rebalance.begin(session, run, plan, now) or \
+                    (run.rebalance or {}).get("state") == "handed":
+                return run.state            # a manager is spreading services back out
+            _end(run, *rebalance.outcome(run), now=now)
         return run.state
     return run.state
 
@@ -262,7 +266,7 @@ def _skip(run, host_id: str, reason: str) -> None:
     run.host_ids = json.dumps(hosts)
 
 
-def _end(run, state: str, now: datetime, detail: str | None) -> None:
+def _end(run, state: str, detail: str | None = None, *, now: datetime) -> None:
     run.state, run.finished_at, run.settle_since = state, now, None
     if detail:
         run.detail = detail[:maintenance.DETAIL_MAX]
@@ -289,6 +293,8 @@ async def tick(session, now: datetime) -> list[str]:
         plan = await session.get(HostMaintenancePlan, run.plan_id)
         if plan is None:
             continue
+        if rebalance.check_deadline(run, now):
+            await rebalance.finish(session, run, now)
         before = run.state
         after = await advance(session, run, plan, now)
         if after != before: