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-pvc-patch.yaml

@@ -1,15 +1,12 @@
-# apiVersion: v1
-# kind: PersistentVolumeClaim
-# metadata:
-#   name: mysql-data-pvc
-# spec:
-#   storageClassName: manual
-#   volumeName: mysql-pv
-
-
 apiVersion: v1
 kind: PersistentVolumeClaim
 metadata:
   name: mysql-data-pvc
 spec:
-  storageClassName: local-path
+  storageClassName: manual
+  volumeName: mysql-pv
+  accessModes:
+    - ReadWriteOnce
+  resources:
+    requests:
+      storage: 5Gi