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/frontend/configmap.yaml

@@ -14,11 +14,6 @@ data:
         root /usr/share/nginx/html;
         index index.html;
 
-        # Serve static files
-        location / {
-            try_files $uri $uri/ /index.html;
-        }
-
         # Proxy API requests to backend service
         location /api/ {
             proxy_pass http://backend:3001;
@@ -27,5 +22,23 @@ data:
             proxy_set_header Connection 'upgrade';
             proxy_set_header Host $host;
             proxy_cache_bypass $http_upgrade;
+            proxy_read_timeout 60s;
+        }
+
+        # Proxy Socket.io (real-time notifications)
+        location /socket.io/ {
+            proxy_pass http://backend:3001;
+            proxy_http_version 1.1;
+            proxy_set_header Upgrade $http_upgrade;
+            proxy_set_header Connection "upgrade";
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_cache_bypass $http_upgrade;
+            proxy_read_timeout 3600s;
+        }
+
+        # Serve static files — React SPA catch-all
+        location / {
+            try_files $uri $uri/ /index.html;
         }
     }