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/overlays/au-syd1/cnpg-system/kustomization.yaml

@@ -0,0 +1,14 @@
+---
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+  - ../../../base/cnpg-system
+
+helmCharts:
+  - name: cloudnative-pg
+    repo: https://cloudnative-pg.github.io/charts
+    version: "0.27.0"
+    releaseName: cloudnative-pg-operator
+    namespace: cnpg-system
+    valuesFile: values.yaml

apps/overlays/au-syd1/cnpg-system/values.yaml

@@ -0,0 +1,16 @@
+replicaCount: 2
+
+resources:
+  limits:
+    cpu: 500m
+    memory: 400Mi
+  requests:
+    cpu: 10m
+    memory: 200Mi
+
+podDisruptionBudget:
+  enabled: true
+  minAvailable: 1
+
+monitoring:
+  createPodMonitor: false # Requires Prometheus Operator CRDs.