feat(swarm): services short, nodes down and managers lost are alertable (#5788)

On 2026-10-14 nine swarm services sat at 0/1 and nothing fired. From each
manager's swarm report the docker plugin now records three figures on
resource "swarm": swarm_services_short, swarm_nodes_down and
swarm_managers_unreachable. Drained nodes are left out of the last two,
since a drain is deliberate. The figures are recorded at most once every
30 s whichever manager reported, and are offered to alert rules.

Agent 1.19.0 sends each manager's ManagerStatus.Reachability. Older agents
fall back to the node's status.

A maintenance job on a swarm node also keeps a window over docker/swarm,
so its drain and reboot hold those alerts until the job ends.

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

plugins/docker/ingest.py

@@ -292,6 +292,20 @@ async def _persist_status(session, host, status: dict) -> None:
     row.reported_at = status["at"]
 
 
+async def _record_swarm_health(session, swarm: dict) -> None:
+    """The swarm-wide figures alert rules watch (#5788), at most once every
+    RECORD_EVERY whichever manager reported."""
+    from datetime import timezone
+    from steward.core.alerts import MetricBatch
+    from . import swarm_health
+    if not swarm_health.due(datetime.now(timezone.utc)):
+        return
+    batch = MetricBatch()
+    for metric, value in swarm_health.figures(swarm).items():
+        batch.add(source_module="docker", resource_name=swarm_health.RESOURCE, metric_name=metric, value=value)
+    await batch.record(session)
+
+
 async def _expire_swarm(session, host) -> None:
     """Drop the swarm rows a host reported while it was a manager."""
     from .models import DockerSwarmNode, DockerSwarmService
@@ -338,6 +352,7 @@ async def persist_host_docker(session, host, snapshots, swarm=None, disk=None,
         await mark("status")
     if swarm is not None:
         await _persist_swarm(session, host, swarm)
+        await _record_swarm_health(session, swarm)
         await mark("swarm")
     if disk is not None:
         await _persist_disk(session, host, disk)