fix: increase backend memory limit and add rollout failure diagnostics

Backend was OOMKilled during rolling update startup (Node.js + Socket.io +
MySQL pool exceeds 256Mi). Raised limit to 512Mi and request to 256Mi.

Jenkinsfile: show kubectl get pods immediately after apply so pod state
is visible in build logs. Added full diagnostics (describe + logs) in
post.failure block so the root cause of any future rollout failure is
visible without needing to SSH into the cluster.

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

k8s/base/backend/deployment.yaml

@@ -64,10 +64,10 @@ spec:
           resources:
             requests:
               cpu: 100m
-              memory: 128Mi
+              memory: 256Mi
             limits:
               cpu: 500m
-              memory: 256Mi
+              memory: 512Mi
           livenessProbe:
             httpGet:
               path: /api/health