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

package.json

@@ -5,7 +5,14 @@
   "scripts": {
     "start": "node index.js"
   },
-  "keywords": ["dashboard", "education", "class", "self-hosted", "express", "sqlite"],
+  "keywords": [
+    "dashboard",
+    "education",
+    "class",
+    "self-hosted",
+    "express",
+    "sqlite"
+  ],
   "author": "lulinretrograde",
   "license": "MIT",
   "repository": {
@@ -25,6 +32,7 @@
     "multer": "^2.1.1",
     "otplib": "^13.4.0",
     "qrcode": "^1.5.4",
-    "resend": "^6.12.0"
+    "resend": "^6.12.0",
+    "web-push": "^3.6.7"
   }
 }