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/devtools/kustomization.yaml

@@ -7,6 +7,8 @@ resources:
   - namespace.yaml
   - vault-secrets.yaml
   - gitea-theme-cm.yaml
+  - gitea-servicemonitor.yaml
+  - gitea-alertrules.yaml
 
 helmCharts:
   # helm repo add gitea-charts https://dl.gitea.com/charts/