Add Helm-based ArgoCD Applications for platform operators

- Longhorn: Helm chart v1.7.2 (sync-wave -2, installs first)
- cert-manager: Helm chart v1.16.3 with CRDs enabled
- CloudNativePG: Helm chart v0.23.0
- Sealed Secrets: Helm chart v2.16.2
- Remove custom StorageClass (Helm chart manages it)

Previously only config resources were deployed without the actual
operators, causing PVCs to pend and CRDs to be missing.

infra/kubernetes/platform/cert-manager/application.yaml

@@ -0,0 +1,26 @@
+apiVersion: argoproj.io/v1alpha1
+kind: Application
+metadata:
+  name: cert-manager-helm
+  namespace: argocd
+  annotations:
+    argocd.argoproj.io/sync-wave: "-1"
+spec:
+  project: default
+  source:
+    repoURL: https://charts.jetstack.io
+    chart: cert-manager
+    targetRevision: v1.16.3
+    helm:
+      valuesObject:
+        crds:
+          enabled: true
+  destination:
+    server: https://kubernetes.default.svc
+    namespace: cert-manager
+  syncPolicy:
+    automated:
+      prune: true
+      selfHeal: true
+    syncOptions:
+      - CreateNamespace=true