fix(alerts): an alert on a count says how many of what, and names them (#6400)

"swarm services short on swarm is 6." becomes "6 swarm services are
short of their replicas.", with a Which line naming each service and how
many of its tasks run. The swarm figures get labels, and the counting
figures (swarm, mounts, drives, updates) read as sentences in the email,
its subject and the Alerts page. A plugin names what its count counts
through core.alerts.names_behind; the Docker plugin does for the three
swarm figures, from the stored swarm. Webhooks get alert.which too.

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

plugins/docker/__init__.py

@@ -52,6 +52,11 @@ def setup(app: "Quart") -> None:
     # A host's agent carries its containers' figures: they go silent with it (#5721).
     from steward.core.alerts import agent_fed
     agent_fed("docker", lambda resource: resource.split("/", 1)[0])
+    # An alert on a swarm figure names the services or nodes it counts (#6400).
+    from steward.core.alerts import names_behind
+    from .swarm_health import NAMERS
+    for metric, names in NAMERS.items():
+        names_behind("docker", metric, names)
     # Deploys and crash-loops go on the timeline.
     from .timeline import register as register_timeline
     register_timeline()