feat(alerting): Phase 1 paging rules — alert-rule catalog (#80) (#26)

Author the Phase-1 (severity=page) subset of the Grafana alert-rule catalog
for shared01 core services as grafana_rule_group resources in
terraform/alerting-catalog-paging.tf. Each rule's labels.severity="page"
routes via the EXISTING notification policy child route to the
pagerduty-platform-infra PagerDuty contact point; the policy/contact points
are NOT modified.

Every PromQL selector was validated against live VictoriaMetrics
(P4169E866C3094E38) before authoring. Pattern mirrors alerting-euc1-dr.tf:
A (query) -> B (reduce, __expr__) -> C (threshold, __expr__), per-service
folders, disable_provenance=true.

18 rules across kubernetes / traefik / victoriametrics / keycloak /
teleport / argocd / eso. Does NOT re-author the victoria-metrics-k8s-stack
chart's generic kube/node/VM alerts (D1); only an outage-grade subset is
promoted to page (D2 fork a). Dropped TraefikNoHealthyBackends
(traefik_service_server_up emits 0 series on shared01).

Refs: thunderbird/platform-infrastructure#80

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: aatchison