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/overlays/on-premise/mysql-pv.yaml

@@ -0,0 +1,29 @@
+apiVersion: v1
+kind: PersistentVolume
+metadata:
+  name: mysql-pv
+  labels:
+    type: local
+spec:
+  storageClassName: manual
+  capacity:
+    storage: 5Gi
+  accessModes:
+    - ReadWriteOnce
+  persistentVolumeReclaimPolicy: Retain
+  hostPath:
+    path: "/mnt/data/mysql"
+    # DirectoryOrCreate: kubelet will create the dir if it doesn't exist,
+    # preventing MySQL CrashLoopBackOff due to missing mount path.
+    type: DirectoryOrCreate
+  # IMPORTANT for multi-node clusters: uncomment nodeAffinity and set your
+  # node's hostname so MySQL always schedules on the node that has the data.
+  # Run: kubectl get nodes  to find the hostname.
+  # nodeAffinity:
+  #   required:
+  #     nodeSelectorTerms:
+  #       - matchExpressions:
+  #           - key: kubernetes.io/hostname
+  #             operator: In
+  #             values:
+  #               - YOUR-NODE-HOSTNAME