feat: deploy n8n on the cluster

The runtime for the estate's autonomous agents. Postgres as a StatefulSet with
its own claim, n8n behind a cert-manager certificate and the nginx ingress, and
both secrets in Vault behind an AppRole scoped to n8n/* alone.

The certificate is issued into the cluster rather than copied into the repo, so
the private key never reaches git. The CA certificate is committed and mounted
as NODE_EXTRA_CA_CERTS, so a workflow can reach keycloak and the domain
gateways without anyone disabling verification to make it work.

One replica and strategy: Recreate, both deliberate — n8n needs Redis and queue
mode before a second scheduler is safe, and a ReadWriteOnce volume deadlocks a
rolling update.

N8N_ENCRYPTION_KEY is generated once and kept in Vault. Regenerating it does not
re-encrypt the stored credentials, it orphans them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

k8s/ingress/certificate.yaml

@@ -0,0 +1,17 @@
+# cert-manager issues this into the `n8n-tls` Secret, from the same
+# ClusterIssuer as the rest of the estate.
+#
+# Deliberately not a copy of a certificate committed to git: the private key
+# stays in the cluster, is rotated by cert-manager, and never reaches a repo.
+apiVersion: cert-manager.io/v1
+kind: Certificate
+metadata:
+  name: n8n-tls
+  namespace: n8n
+spec:
+  secretName: n8n-tls
+  dnsNames:
+    - n8n.sttlab.local
+  issuerRef:
+    name: sttlab-local-ca
+    kind: ClusterIssuer