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/secrets/externalsecret.yaml

@@ -0,0 +1,22 @@
+# Two values, both of which must outlive any pod.
+#
+# N8N_ENCRYPTION_KEY encrypts the credentials n8n stores in its database. Lose
+# it and every stored credential becomes unreadable — the workflows survive, the
+# credentials do not. It is generated once and kept in Vault for that reason,
+# never regenerated on a redeploy.
+apiVersion: external-secrets.io/v1
+kind: ExternalSecret
+metadata:
+  name: n8n-secrets
+  namespace: n8n
+spec:
+  refreshInterval: 1h
+  secretStoreRef:
+    kind: ClusterSecretStore
+    name: vault-n8n
+  target:
+    name: n8n-secrets
+    creationPolicy: Owner
+  dataFrom:
+    - extract:
+        key: n8n/instance