feat: add MetalLB for on-premise LoadBalancer support

- Add MetalLB IPAddressPool (192.168.108.213/32) and L2Advertisement
  so the frontend gets a stable external IP on the LAN
- Change frontend service type: NodePort → LoadBalancer
- Add 'Setup MetalLB' stage in Jenkinsfile that installs MetalLB v0.14.8
  (idempotent) and applies the IP pool config before each deploy

After deploy: kubectl get svc frontend -n scrum-manager
should show EXTERNAL-IP: 192.168.108.213
App accessible at: http://192.168.108.213

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

k8s/base/frontend/service.yaml

@@ -6,7 +6,7 @@ metadata:
     app.kubernetes.io/name: frontend
     app.kubernetes.io/component: web
 spec:
-  type: NodePort
+  type: LoadBalancer
   ports:
     - port: 80
       targetPort: 80