feat(iam): wire Zitadel into namespace iam under ArgoCD

Coexistence wiring for the Authentik replacement: ZITADEL gets its own CNPG
cluster, SOPS masterkey, Helm values and ArgoCD Application in namespace iam.
Authentik keeps serving; no consumer is moved and no Authentik object changes.

- k8s/infra/databases/zitadel-db.yaml: CNPG Postgres 16.2, 3 instances
- k8s/infra/iam/zitadel-values.yaml: chart 10.4.0 pinned, DSN from the CNPG
  secret, masterkey from a Secret, metrics ServiceMonitor, second-sync caveat
- k8s/argocd/apps/30-security.yaml: Application named zitadel, wave 3,
  automated sync deliberately absent for the first sync (chart PreSync hook
  re-run risk)
- k8s/argocd/secrets/zitadel-masterkey.enc.yaml: 32-byte masterkey, SOPS
- k8s/infra/monitoring/alerts/svc-zitadel-rules.yaml: target-down, login-UI,
  crash-loop and PreSync-job-failed rules
- terraform/files/coredns/Corefile: in-cluster rewrite for the zitadel host

Not applied. The cluster is mid-incident: vault-0 and authentik-server are 0/1,
authentik-db-2 has 14 restarts, prometheus-0 is 1/2 CrashLoopBackOff and
kube-state-metrics has 1925 restarts. Two manual steps still gate the first
sync: the Cloudflare Tunnel public hostname plus a proxied CNAME, and
terraform apply + make apply-cp for the CoreDNS rewrite (Talos-owned).

The offline masterkey copy is at ~/zitadel-masterkey.offline (mode 600). Move it
to sealed storage and delete it before the first sync. Losing it makes every
encrypted ZITADEL value unreadable.

k8s/argocd/secrets/secret-generator.yaml

@@ -38,3 +38,4 @@ files:
     - forgejo-registry-token.enc.yaml
     - pi-sandbox-secrets.enc.yaml
     - etcd-defrag-creds.enc.yaml
+  - zitadel-masterkey.enc.yaml

k8s/argocd/secrets/zitadel-masterkey.enc.yaml

@@ -0,0 +1,23 @@
+apiVersion: v1
+kind: Secret
+metadata:
+    name: zitadel-masterkey
+    namespace: iam
+type: Opaque
+stringData:
+    masterkey: ENC[AES256_GCM,data:dGp0wSCUJORP6W5PHLKYaQEqrZsgiAwU1GqcuvuZV+g=,iv:Wwt6E/XXwsoVEQxe4gG1m5oJLOcNUXngftADQvZQepw=,tag:1Vget5gs+t455U9zLavdwg==,type:str]
+sops:
+    age:
+        - enc: |
+            -----BEGIN AGE ENCRYPTED FILE-----
+            YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSBld0tUWHNWem13cDlxRG14
+            QWRlTjA5SzZnS2pBd2M0WWkzQWxKZW9HZWdJCk00QWJJcWZFM1ZTSUlReUlGZG4z
+            RFBSUWRlVUx2NGN2OFAwazBYRTVVNkEKLS0tIHpSSW9IWHhNa0d3SmEydXU4akVj
+            MGJkN2U4WmtLbVFMZm55U1hEZElZMlUKKV9twnC9GC7MSmNLxEBHbPl4RN5Hsd+z
+            NViCKxUoQAB8apn2XVHZcDnyfKNfr1P3aSa3IIHyVY3prgo1OFDpdw==
+            -----END AGE ENCRYPTED FILE-----
+          recipient: age1e5fq3hwxy78psus2nfvmtmua36g0u3suk78ephw6246l974d2utsvn0hla
+    encrypted_regex: ^(data|stringData)$
+    lastmodified: "2026-10-24T08:57:27Z"
+    mac: ENC[AES256_GCM,data:hXDAddVmnlf/yinY2c2Q8yfsjqi6n9cRbEbwykqYScd6mDPfdF7BriTiu9K+CV/IvIT4zcMTZbhBeQ/vTCAh0TZDp1lOVglwtNH/imgagyzPmcCvbCIkZKwF5vwWQIGHcDI6pk4/zVQkqt7zKTf8+Qj0pNh5UrAjVeEFE7cuJkQ=,iv:oIQvROFGbiESmMTthHOJiNM+Gy5BCfM5XtDWbszOPmc=,tag:AaYrJrwXuKD7w0p+TTa7FA==,type:str]
+    version: 3.13.2