v2.7.0

Split metrics-stack into per-component packages

metrics-stack is now a metapackage over four new packages so partial
deployments work (Prometheus without Grafana, Grafana on another host,
alerting-free stacks):

- metrics-stack-common: the shared 'metrics' Podman network
- metrics-stack-prometheus: Prometheus, rules, targets.d, monitoring-add-exporter
- metrics-stack-alertmanager: Alertmanager, monitoring-configure-email
- metrics-stack-grafana: Grafana, provisioning, monitoring-add-dashboard

Alertmanager now registers its scrape target via monitoring-add-exporter
(the exporter-package pattern) instead of being hardcoded in prometheus.yml,
so Prometheus-only hosts don't carry a permanently-down target.

Clean break: no Replaces/Conflicts migration plumbing -- pre-split installs
must uninstall the old metrics-stack before installing these (data under
/var/lib survives either way).

CI: stack debs install as one transaction, alertmanager auto-registration
is asserted, and a new step removes Alertmanager and confirms Prometheus
and Grafana keep running without it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>