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>

docs/core/alerting.md

@@ -213,6 +213,7 @@ Configured at `/settings/` under Webhook. The body is a Jinja2 template rendered
 | `alert.threshold` | float | Configured threshold |
 | `alert.resource` | str | Resource name |
 | `alert.source_module` | str | `ping`, `dns`, `traefik`, etc. |
+| `alert.which` | str | For a figure that counts things, what it counts when it fires, joined by ", " (`web_app (0 of 2 running), …`); `raw.which` is the list. Empty for other figures. |
 | `alert.timestamp` | str | ISO 8601 UTC |
 
 Default template (Discord-compatible):