feat: Web Push notifications

- VAPID-based push via web-push package
- push_subscriptions table: endpoint + keys per user (upsert on conflict)
- GET /api/push/vapid-key — public key for subscribe flow
- POST/DELETE /api/push/subscribe — store/remove subscription
- POST /api/push/test — manual test notification
- Hourly scheduler: notifies users day before homework due + countdown expires
- SW: push event handler shows notification; notificationclick opens /app
- Settings: Push section with enable/disable/test buttons, auto-detects
  browser support and VAPID availability

public/sw.js

@@ -28,6 +28,32 @@ self.addEventListener('activate', e => {
   );
 });
 
+self.addEventListener('push', e => {
+  let data = { title: 'Klassenportal', body: '' };
+  try { data = e.data ? e.data.json() : data; } catch {}
+  e.waitUntil(
+    self.registration.showNotification(data.title, {
+      body: data.body,
+      icon: '/icons/icon-192.png',
+      badge: '/icons/icon-192.png',
+      tag: data.tag || 'kp',
+      renotify: true,
+      data: { url: data.url || '/app' },
+    })
+  );
+});
+
+self.addEventListener('notificationclick', e => {
+  e.notification.close();
+  const url = e.notification.data?.url || '/app';
+  e.waitUntil(
+    clients.matchAll({ type: 'window', includeUncontrolled: true }).then(list => {
+      const found = list.find(c => c.url.includes(self.location.origin));
+      return found ? found.focus() : clients.openWindow(url);
+    })
+  );
+});
+
 self.addEventListener('fetch', e => {
   const url = new URL(e.request.url);