feat: migrate cnpg-system from Terraform to ArgoCD

- Add cnpg-system base ArgoCD application with namespace
- Create cnpg-system overlay for au-syd1 with CloudNativePG Helm chart
- Update platform ApplicationSet to include cnpg-system deployment
- Configure cloudnative-pg operator v0.27.0 with HA and resource limits
- Maintain one-to-one migration from Terraform configuration

argocd/applicationsets/platform.yaml

@@ -11,6 +11,7 @@ spec:
       revision: HEAD
       directories:
       - path: apps/overlays/*/artifactapi
+      - path: apps/overlays/*/cnpg-system
       - path: apps/overlays/*/reflector-system
       - path: apps/overlays/*/reloader-system
       - path: apps/overlays/*/jfrog