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/monitoring/kustomization.yaml

@@ -7,7 +7,6 @@ kind: Kustomization
 resources:
   - alerts/ingress-alerts.yaml
   - alerts/svc-argocd-rules.yaml
-  - alerts/svc-authentik-rules.yaml
   - alerts/svc-zitadel-rules.yaml
   - alerts/svc-forgejo-rules.yaml
   - alerts/svc-grafana-rules.yaml
@@ -17,7 +16,6 @@ resources:
   - alerts/api-gateway-alerts.yaml
   - alerts/cag-kv-cache-rules.yaml
   - servicemonitors/argocd.yaml
-  - servicemonitors/authentik.yaml
   - servicemonitors/forgejo.yaml
   - servicemonitors/minio.yaml
   - servicemonitors/ingress-nginx.yaml