Deploy k8up backup operator in platform ArgoCD project

The k8up CRDs already exist in the cluster (managed by a separate k8up-crds
Helm release) but the operator itself was never deployed. Install it following
the cnpg-system pattern so k8up backup/restore custom resources are actually
reconciled.

- Add apps/base/k8up-system Namespace + kustomization
- Add au-syd1 overlay inflating the k8up chart 4.10.0 from the artifactapi
  virtual-helm proxy (replicaCount 2 with leader election, PDB minAvailable 1,
  sane operator resource requests/limits, ServiceMonitor/PrometheusRule off)
- Do not emit the chart CRDs: they live in the chart crds/ directory which the
  kustomize helm inflator skips, leaving CRD ownership with the existing
  k8up-crds Helm release so ArgoCD does not fight over them
- Register apps/overlays/*/k8up-system in the platform ApplicationSet

apps/base/k8up-system/kustomization.yaml

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

apps/base/k8up-system/namespace.yaml

@@ -0,0 +1,5 @@
+---
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: k8up-system