v2.8.0

Add Alertmanager HA clustering support (monitoring-configure-cluster)

Gossip runs over TCP+UDP 9094. The packaged Quadlet unit becomes
cluster-ready but inert: a small entrypoint wrapper (riding the existing
/etc/alertmanager bind mount) appends flags from an unowned
/etc/alertmanager/cluster.args, so the conffile unit is never edited and
Image-bump upgrades stay clean. Quadlet drop-ins would have been the
natural mechanism but need Podman >= 5.0, and Ubuntu 24.04 ships 4.9.

monitoring-configure-cluster writes the cluster flags, opens 9094 tcp+udp
on the firewall, restarts the service, and prints cluster status;
--disable reverses all of it.

Prometheus's alerting config moves from a static alertmanager:9093 target
to file_sd over /etc/prometheus/alertmanagers.d/ (the targets.d pattern):
the alertmanager package registers the local instance, and
monitoring-configure-cluster registers peers so alerts fan out to every
cluster member. 9094 is always published on the host but only
firewall-opened when clustering is enabled.

CI: new smoke step enables clustering against an unreachable TEST-NET
peer, asserts the running process carries the cluster flags, that the
cluster API answers, and that Prometheus discovers both alertmanagers,
then disables and asserts cleanup.

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