fix: k8s on-premise deployment and session persistence

Database fixes:
- Add hostPath.type=DirectoryOrCreate so kubelet auto-creates /mnt/data/mysql
- Add fsGroup=999 so MySQL process can write to the hostPath volume
- Add MYSQL_ROOT_HOST=% to allow backend pods to authenticate as root
- Fix liveness/readiness probes to include credentials (-p$MYSQL_ROOT_PASSWORD)
- Increase probe initialDelaySeconds (30/60s) for slow first-run init
- Add 15s grace sleep in backend initContainer after MySQL TCP is up
- Add persistentVolumeReclaimPolicy=Retain to prevent accidental data loss
- Explicit accessModes+resources in PVC patch to avoid list merge ambiguity
- Add nodeAffinity comment in PV for multi-node cluster guidance

Ingress/nginx fixes:
- Remove broken rewrite-target=/ that was rewriting all paths (incl /api) to /
- Route /socket.io directly to backend for WebSocket support
- Add /socket.io/ proxy location to both nginx.conf and K8s ConfigMap

Frontend fix:
- Persist currentUser to localStorage on login so page refresh no longer
  clears session and redirects users back to the login page

Tooling:
- Add k8s/overlays/on-premise/deploy.sh for one-command deployment

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

k8s/base/backend/deployment.yaml

@@ -24,14 +24,14 @@ spec:
             - sh
             - -c
             - |
-              echo "Waiting for MySQL port to open..."
+              echo "Waiting for MySQL TCP to be available..."
               until nc -z mysql 3306; do
-                echo "MySQL not ready yet, retrying in 5s..."
-                sleep 5
+                echo "MySQL not reachable yet, retrying in 3s..."
+                sleep 3
               done
-              echo "Port open — waiting 15s for MySQL to finish initializing..."
+              echo "MySQL TCP is up. Waiting 15s for full initialization..."
               sleep 15
-              echo "MySQL is ready!"
+              echo "Proceeding to start backend."
       containers:
         - name: backend
           image: scrum-backend:latest
@@ -48,12 +48,12 @@ spec:
               valueFrom:
                 secretKeyRef:
                   name: mysql-secret
-                  key: MYSQL_USER        # matches new secret key
+                  key: DB_USER
             - name: DB_PASSWORD
               valueFrom:
                 secretKeyRef:
                   name: mysql-secret
-                  key: MYSQL_PASSWORD    # matches new secret key
+                  key: DB_PASSWORD
             - name: DB_NAME
               valueFrom:
                 secretKeyRef:
@@ -72,7 +72,7 @@ spec:
             httpGet:
               path: /api/health
               port: http
-            initialDelaySeconds: 30
+            initialDelaySeconds: 15
             periodSeconds: 10
             timeoutSeconds: 3
             failureThreshold: 3
@@ -80,7 +80,7 @@ spec:
             httpGet:
               path: /api/health
               port: http
-            initialDelaySeconds: 15
+            initialDelaySeconds: 5
             periodSeconds: 5
             timeoutSeconds: 3
-            failureThreshold: 5
+            failureThreshold: 5