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>

frontend/vite.config.ts

@@ -7,6 +7,9 @@ export default defineConfig({
   plugins: [
     react(),
     VitePWA({
+      strategies: 'injectManifest',
+      srcDir: 'src',
+      filename: 'sw.js',
       registerType: 'autoUpdate',
       manifest: {
         name: 'Maintenance',
@@ -21,9 +24,7 @@ export default defineConfig({
           { src: '/maintenance/icons/icon-512.png', sizes: '512x512', type: 'image/png' },
         ],
       },
-      workbox: {
-        navigateFallback: '/maintenance/index.html',
-        navigateFallbackDenylist: [/\/api\//],
+      injectManifest: {
         globPatterns: ['**/*.{js,css,html,ico,png,svg}'],
       },
     }),