refactor: make this a reusable deployment, not ours

Every environment value is a ${VAR} for the caller to substitute: hostname,
images, storage, ingress class, TLS issuer, Vault server and path, timezone,
retention. Nothing in k8s/ names a host, a realm, an agent or an estate.

Removed: the Flux plumbing under dev/, which described one deployment, and the
ExternalSecret pulling a specific agent's Keycloak client secret. Both belong to
whoever deploys this. The lab CA ConfigMap goes with them — the mount stays,
parameterised and marked optional, because 'trust a private CA' is a property of
an environment and not of n8n.

The README is now the template's: how to consume it, what every variable means,
and the three things it deliberately does not do.

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

k8s/n8n/deployment.yaml

@@ -2,7 +2,7 @@ apiVersion: apps/v1
 kind: Deployment
 metadata:
   name: n8n
-  namespace: n8n
+  namespace: ${NAMESPACE}
 spec:
   # One replica. n8n's queue mode splits the editor from workers and needs Redis
   # to do it; without that, a second replica is two schedulers racing over the
@@ -26,7 +26,7 @@ spec:
         fsGroup: 1000
       initContainers:
         - name: wait-for-postgres
-          image: postgres:16.10-alpine
+          image: ${POSTGRES_IMAGE}
           command:
             - sh
             - -c
@@ -36,7 +36,7 @@ spec:
               done
       containers:
         - name: n8n
-          image: docker.n8n.io/n8nio/n8n:2.37.4
+          image: ${N8N_IMAGE}
           ports:
             - name: http
               containerPort: 5678
@@ -64,24 +64,27 @@ spec:
                 secretKeyRef:
                   name: n8n-secrets
                   key: N8N_ENCRYPTION_KEY
-            # The lab CA, so a workflow can call keycloak.sttlab.local and the
-            # domain gateways without disabling verification. n8n is Node, hence
-            # this variable rather than a trust store.
+            # A private CA to trust, so a workflow can call an internal
+            # endpoint over TLS without anyone disabling verification to make it
+            # work. n8n is Node, hence this variable rather than a trust store.
+            # Provided by the deployment; an empty ConfigMap disables it in
+            # practice, since the file will not exist and Node falls back to its
+            # own bundle.
             - name: NODE_EXTRA_CA_CERTS
-              value: /etc/sttlab-ca/ca.crt
+              value: /etc/extra-ca/ca.crt
           envFrom:
             - configMapRef:
                 name: n8n-config
-            # The agents' Keycloak client secrets, reached from a workflow as
-            # `$env.APPROVISIONNEMENT_CLIENT_SECRET`. A workflow acts as its
-            # agent, so it is the workflow that must be able to exchange tokens.
+            # Whatever else this deployment's workflows need in their
+            # environment. Provided by the deployment, not by this repo: an
+            # empty Secret of that name is a valid answer.
             - secretRef:
-                name: n8n-agent-secrets
+                name: ${EXTRA_ENV_SECRET}
           volumeMounts:
             - name: data
               mountPath: /home/node/.n8n
-            - name: sttlab-ca
-              mountPath: /etc/sttlab-ca
+            - name: extra-ca
+              mountPath: /etc/extra-ca
               readOnly: true
           readinessProbe:
             httpGet:
@@ -110,6 +113,7 @@ spec:
         - name: data
           persistentVolumeClaim:
             claimName: n8n-data
-        - name: sttlab-ca
+        - name: extra-ca
           configMap:
-            name: sttlab-ca
+            name: ${EXTRA_CA_CONFIGMAP}
+            optional: true