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/ingress.yaml

@@ -4,12 +4,25 @@ metadata:
   name: scrum-manager-ingress
   annotations:
     kubernetes.io/ingress.class: nginx
-    nginx.ingress.kubernetes.io/rewrite-target: /
+    # No rewrite-target here — the old global rewrite-target: / was
+    # rewriting every path (including /api/tasks) to just /, breaking the API.
+    nginx.ingress.kubernetes.io/proxy-read-timeout: "3600"
+    nginx.ingress.kubernetes.io/proxy-send-timeout: "3600"
 spec:
   rules:
     - host: scrum.local
       http:
         paths:
+          # Socket.io long-polling and WebSocket connections go directly to backend.
+          - path: /socket.io
+            pathType: Prefix
+            backend:
+              service:
+                name: backend
+                port:
+                  number: 3001
+          # All other traffic (including /api/) goes to frontend nginx,
+          # which proxies /api/ to backend internally. This avoids double-routing.
           - path: /
             pathType: Prefix
             backend:
@@ -17,10 +30,3 @@ spec:
                 name: frontend
                 port:
                   number: 80
-          - path: /api
-            pathType: Prefix
-            backend:
-              service:
-                name: backend
-                port:
-                  number: 3001