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/postgres/statefulset.yaml

@@ -0,0 +1,71 @@
+# A StatefulSet rather than a Deployment: one identity, one volume, and no
+# window where two pods hold the same data directory. The volumeClaimTemplate
+# also means the claim is not deleted with the workload — losing the workflows
+# to a `kubectl delete deploy` would be an avoidable way to lose them.
+apiVersion: apps/v1
+kind: StatefulSet
+metadata:
+  name: n8n-postgres
+  namespace: n8n
+spec:
+  serviceName: n8n-postgres
+  replicas: 1
+  selector:
+    matchLabels:
+      app: n8n-postgres
+  template:
+    metadata:
+      labels:
+        app: n8n-postgres
+    spec:
+      securityContext:
+        fsGroup: 999
+      containers:
+        - name: postgres
+          image: postgres:16.10-alpine
+          ports:
+            - name: postgres
+              containerPort: 5432
+          env:
+            - name: POSTGRES_USER
+              value: n8n
+            - name: POSTGRES_DB
+              value: n8n
+            - name: POSTGRES_PASSWORD
+              valueFrom:
+                secretKeyRef:
+                  name: n8n-secrets
+                  key: DB_PASSWORD
+            # The image initialises into this directory. Naming a subdirectory
+            # of the mount rather than the mount itself keeps initdb away from
+            # the lost+found some volume types leave at the root.
+            - name: PGDATA
+              value: /var/lib/postgresql/data/pgdata
+          volumeMounts:
+            - name: data
+              mountPath: /var/lib/postgresql/data
+          readinessProbe:
+            exec:
+              command: ["pg_isready", "-U", "n8n", "-d", "n8n"]
+            initialDelaySeconds: 5
+            periodSeconds: 10
+          livenessProbe:
+            exec:
+              command: ["pg_isready", "-U", "n8n", "-d", "n8n"]
+            initialDelaySeconds: 30
+            periodSeconds: 20
+          resources:
+            requests:
+              cpu: 50m
+              memory: 256Mi
+            limits:
+              cpu: "1"
+              memory: 1Gi
+  volumeClaimTemplates:
+    - metadata:
+        name: data
+      spec:
+        accessModes: ["ReadWriteOnce"]
+        resources:
+          requests:
+            storage: 5Gi