feat: migrate cnpg-system from Terraform to ArgoCD (#44)

- 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

Reviewed-on: #44

apps/base/cnpg-system/kustomization.yaml

@@ -0,0 +1,6 @@
+---
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+  - namespace.yaml

apps/base/cnpg-system/namespace.yaml

@@ -0,0 +1,5 @@
+---
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: cnpg-system