Add Web Push notifications for task assignment

When a task is created or reassigned to a staff member, a push notification
is sent to any devices they have subscribed on. Keys are auto-generated on
first boot and stored in the config table (no manual VAPID setup needed).

- Backend: web-push dep, push_subscriptions table, lib/push.js (VAPID +
  send), routes/push.js (vapid-key / subscribe / unsubscribe endpoints),
  push notify wired into task-core createTask and tasks PATCH reassignment
- Frontend: switched vite-plugin-pwa to injectManifest strategy, custom
  sw.js handles precache + push event + notificationclick, usePushSubscription
  hook requests permission and registers subscription on login

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

backend/package.json

@@ -14,6 +14,7 @@
     "fastify": "^4.28.1",
     "jose": "^5.9.6",
     "nodemailer": "^6.9.16",
-    "pg": "^8.13.1"
+    "pg": "^8.13.1",
+    "web-push": "^3.6.7"
   }
 }