feat: add PrometheusRule alerts for all services

28 alert rules across 9 PrometheusRule files covering infrastructure
(Longhorn, cert-manager), data (PostgreSQL, OpenBao, OpenSearch),
storage (SeaweedFS), devtools (Gitea), identity (Hydra, Kratos),
media (LiveKit), and mesh (Linkerd golden signals for all services).

Severity routing: critical alerts fire to Matrix + email, warnings
to Matrix only (AlertManager config updated in separate commit).

base/media/kustomization.yaml

@@ -6,6 +6,10 @@ namespace: media
 resources:
   - namespace.yaml
   - vault-secrets.yaml
+  - livekit-alertrules.yaml
+  # livekit-servicemonitor.yaml disabled — LiveKit runs on hostNetwork and port 6789
+  # is not reachable from Prometheus due to host firewall. Open port 6789 on the host
+  # or add an iptables rule, then re-enable.
 
 helmCharts:
   # helm repo add livekit https://helm.livekit.io