feat: migrate cattle-system (Rancher) from Terraform to ArgoCD (#39)

- Add cattle-system base ArgoCD application with namespace, Vault integration, and ingress
- Create cattle-system overlay for au-syd1 with Rancher Helm chart configuration
- Update platform ApplicationSet to include cattle-system deployment
- Update platform project to include Rancher Helm repository as source
- Configure Rancher v2.13.1 with HA, TLS, audit logging, and bootstrap secret from Vault
- Maintain one-to-one migration from Terraform configuration

Reviewed-on: #39

apps/base/cattle-system/vaultauth.yaml

@@ -0,0 +1,18 @@
+---
+apiVersion: secrets.hashicorp.com/v1beta1
+kind: VaultAuth
+metadata:
+  name: rancher
+  namespace: cattle-system
+spec:
+  method: kubernetes
+  mount: k8s/au/syd1
+  vaultConnectionRef: vso-system/default
+  allowedNamespaces:
+    - cattle-system
+  kubernetes:
+    role: rancher
+    serviceAccount: rancher
+    audiences:
+      - vault
+    tokenExpirationSeconds: 600