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

argocd/projects/platform.yaml

@@ -13,6 +13,7 @@ spec:
     - https://releases.rancher.com/server-charts/stable
     - https://charts.jetstack.io
     - https://kubernetes-sigs.github.io/external-dns/
+    - https://cloudnative-pg.github.io/charts
   destinations:
     - namespace: '*-system'
       server: https://kubernetes.default.svc