feat(ci): GitLab CI/CD dev→prod pipeline, kustomize base+overlays

Multiple developers now work on this repo, and the only working deploy
trigger required pushing to GitHub - which contradicts the standing
GitLab-canonical policy for this repo - while every actual deploy was a
manual kubectl run against one environment (no prod exists at all).

Restructures deploy/k8s/ into base/ + overlays/{dev,prod}: overlays/dev
is a verified byte-for-byte no-op for the live sandbox (kubectl kustomize
diff against the old flat layout is empty), overlays/prod is scaffolded
but inert (placeholder hostname + JMAP_SERVER_URL, since neither a prod
hostname decision nor a prod Stalwart exist yet). deploy/k8s/ca/ (the
EJBCA internal CA) is untouched and never referenced by either overlay.

Adds .gitlab-ci.yml: verify (MR gate, no push/deploy) -> build+deploy-dev
(automatic on push to dev, one image name/tag-only environments, fixing
the old -dev/-beta naming split) -> promote (manual, protected
`production` environment, retags the exact dev digest via
`docker buildx imagetools create` - never rebuilds - and is left as a
documented TODO for the actual `kubectl apply` until prod is real).

Updates VNCMAIL-SETUP.md and deploy/k8s/README.md to describe the new
flow and correct the aspirational promotion description that assumed a
"production image" CI never actually built.

Also fixes a pre-existing lint error (no-control-regex false positive on
an intentional DN-sanitizing character class in lib/smime-ca/ejbca.ts)
that was blocking this commit's pre-commit hook - unrelated to this
change otherwise, confirmed already present on dev before this branch.

Runner/RBAC/registry setup is an infra prerequisite this commit cannot
provide - documented in the pipeline plan, not part of this diff.

deploy/k8s/base/ingress.yaml

@@ -0,0 +1,33 @@
+# Exposes VNCmail+ at vncmail.sandbox.vnc.de, alongside bulwark.sandbox.vnc.de.
+# MATCH YOUR CLUSTER — inspect the existing Bulwark ingress and copy its
+# ingressClassName + TLS/cert-manager annotations:
+#   kubectl get ingress -A | grep bulwark
+#   kubectl get ingress <bulwark-ingress> -n <ns> -o yaml
+apiVersion: networking.k8s.io/v1
+kind: Ingress
+metadata:
+  name: vncmail-plus
+  annotations:
+    # cert-manager issuer — set to whatever bulwark.sandbox.vnc.de uses.
+    cert-manager.io/cluster-issuer: letsencrypt-prod
+    # Mail attachments can be large; raise the nginx body limit.
+    nginx.ingress.kubernetes.io/proxy-body-size: "100m"
+spec:
+  # microk8s ingress addon class is usually "public" (nginx). Confirm with
+  # `kubectl get ingressclass` and match bulwark's.
+  ingressClassName: public
+  tls:
+    - hosts:
+        - vncmail.sandbox.vnc.de
+      secretName: vncmail-plus-tls
+  rules:
+    - host: vncmail.sandbox.vnc.de
+      http:
+        paths:
+          - path: /
+            pathType: Prefix
+            backend:
+              service:
+                name: vncmail-plus
+                port:
+                  number: 80