chore(iam): retire Authentik

Zitadel is running and has provisioned OIDC clients for all 12 consumers
(state.json: kubernetes, paperless, forgejo, argocd, grafana, immich, minio,
vault, poimen, api-gw, queue-sqs, homarr). Authentik's remaining job was to
serve until the last consumer moved.

Remove every Authentik artifact from GitOps:
- ArgoCD Application `authentik` and its Helm values
- ArgoCD Application `iam-jobs` (provisioner RBAC + migration job)
- ArgoCD Application `oidc-bootstrap` (one-shot apiserver restart)
- authentik-db CNPG cluster definition
- authentik-secrets SOPS file and secret-generator entry
- Ingress rule, cloudflared tunnel hostname, CoreDNS rewrite
- Prometheus alerts and ServiceMonitor
- admin OIDC ClusterRoleBinding provisioning binding
- scripts/iam/provision-rbac.py (the Authentik provisioner)

The chart repo https://charts.goauthentik.io is removed from the AppProject.

Kube-apiserver OIDC and consumer configs are switched to Zitadel in the
local cluster-config (gitignored) and applied separately via talosctl.

k8s/infra/iam/kustomization.yaml

@@ -5,8 +5,6 @@ resources:
   - rbac-dashboard-rolebinding.yaml
 
 # IAM provisioning is manual-only (security-sensitive).
-# Script: scripts/iam/authentik-provision.py
+# Script: scripts/iam/zitadel-provision.py
 # Run:
-#   export AUTHENTIK_BOOTSTRAP_TOKEN=$(kubectl -n iam get secret authentik-secrets \
-#     -o jsonpath='{.data.AUTHENTIK_BOOTSTRAP_TOKEN}' | base64 -d)
-#   python3 scripts/iam/authentik-provision.py
+#   scripts/iam/zitadel-provision.py all