Add Spring Boot sample application for testing failover scenarios

[vlsi]

The PR adds helmfile configuration which deploys pgskipper and a sample application to verify connectivity and failover behavior.

Here's what I get with macOS + OrbStack:

$ USE_LOCAL_IMAGES=true helmfile -e orbstack sync
...
hook[postsync] logs | Waiting for application pods to be ready...
hook[postsync] logs | pod/failover-test-postgresql-failover-test-7c8589c549-6pb45 condition met
hook[postsync] logs | pod/failover-test-postgresql-failover-test-7c8589c549-qhtjw condition met
hook[postsync] logs | Application is ready!
hook[postsync] logs |
hook[postsync] logs | =====================================
hook[postsync] logs | Deployment Complete!
hook[postsync] logs | =====================================
hook[postsync] logs |
hook[postsync] logs | PostgreSQL Cluster Status:
hook[postsync] logs |
hook[postsync] logs | Application Status:
hook[postsync] logs | NAME                                                      READY   STATUS    RESTARTS   AGE
hook[postsync] logs | failover-test-postgresql-failover-test-7c8589c549-6pb45   1/1     Running   0          34s
hook[postsync] logs | failover-test-postgresql-failover-test-7c8589c549-qhtjw   1/1     Running   0          34s
hook[postsync] logs |
hook[postsync] logs | Services:
hook[postsync] logs | NAME                   TYPE        CLUSTER-IP        EXTERNAL-IP   PORT(S)                      AGE
hook[postsync] logs | monitoring-collector   ClusterIP   192.168.194.171   <none>        8000/TCP,9273/TCP            76s
hook[postsync] logs | patroni-headless       ClusterIP   None              <none>        5432/TCP,8008/TCP            61s
hook[postsync] logs | pg-patroni             ClusterIP   192.168.194.203   <none>        5432/TCP,8008/TCP,22/TCP     61s
hook[postsync] logs | pg-patroni-api         ClusterIP   192.168.194.234   <none>        5432/TCP,8008/TCP,22/TCP     31h
hook[postsync] logs | pg-patroni-ro          ClusterIP   192.168.194.178   <none>        5432/TCP,8008/TCP,22/TCP     31h
hook[postsync] logs | postgres-operator      ClusterIP   192.168.194.145   <none>        8383/TCP,8080/TCP,8443/TCP   31h
hook[postsync] logs |
hook[postsync] logs | Next steps:
hook[postsync] logs | 1. Check application logs: kubectl logs -f -n default -l app.kubernetes.io/name=postgresql-failover-test
hook[postsync] logs | 2. Test failover: ./scripts/trigger-failover.sh
hook[postsync] logs | 3. Monitor reconnection: ./scripts/test-reconnection.sh