Install valkey-operator (#365)

Makes the valkey-operator available in the cluster so a later change can replace the self-managed redis Deployment with a Valkey CR. This PR only installs the operator; it does not create any Valkey CR or touch existing apps.

Changes:
- Add `apps/base/valkey-operator-system/` (namespace `valkey-operator-system`, mirroring the cnpg-system pattern).
- Add `apps/overlays/au-syd1/valkey-operator-system/` which inflates the `valkey-operator` 0.5.0 Helm chart from the artifactapi virtual helm repo with `includeCRDs: true` (the chart ships CRDs in the Helm-native `crds/` dir, which `helm template` skips otherwise).
- Route the operator image through artifactapi: `artifactapi.k8s.syd1.au.unkin.net/ghcr/valkey-io/valkey-operator:v0.5.0`.
- Register `apps/overlays/*/valkey-operator-system` in the platform ApplicationSet. No AppProject change needed: the `*-system` destination and the virtual-helm sourceRepo already cover it (same as cnpg-system/cephrgw-system).

Depends on unkin/terraform-artifactapi#29 (adds the valkey helm remote + virtual member and allowlists the operator image). That PR must merge and apply before this one syncs, or the chart pull / image pull will 404/403.

Validated with `kustomize build --enable-helm` against the upstream chart (identical version/values): renders the Namespace, 2 CRDs, operator Deployment (image = the artifactapi ghcr path), RBAC, SA, and metrics Service, all in `valkey-operator-system`.

Reviewed-on: #365
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

argocd/applicationsets/platform.yaml

@@ -43,6 +43,7 @@ spec:
       - path: apps/overlays/*/reloader-system
       - path: apps/overlays/*/reposync
       - path: apps/overlays/*/traefik-system
+      - path: apps/overlays/*/valkey-operator-system
       - path: apps/overlays/*/vm-system
       - path: apps/overlays/*/vpa-system
       - path: apps/overlays/*/vault