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/overlays/on-premise/metallb/ipaddresspool.yaml

@@ -0,0 +1,8 @@
+apiVersion: metallb.io/v1beta1
+kind: IPAddressPool
+metadata:
+  name: scrum-manager-pool
+  namespace: metallb-system
+spec:
+  addresses:
+    - 192.168.108.213/32

k8s/overlays/on-premise/metallb/l2advertisement.yaml

@@ -0,0 +1,8 @@
+apiVersion: metallb.io/v1beta1
+kind: L2Advertisement
+metadata:
+  name: scrum-manager-l2
+  namespace: metallb-system
+spec:
+  ipAddressPools:
+    - scrum-manager-pool