feat: add Docker support for push notifications

- Update docker-compose.yml to include VAPID environment variables
- Create DOCKER_PUSH_NOTIFICATIONS_SETUP.md with Docker-specific instructions
- Create PUSH_NOTIFICATIONS_DOCKER_CHECKLIST.md for easy setup tracking
- Update .env.example with Docker command for generating VAPID keys

Setup for Docker:
1. Generate VAPID keys (npx web-push generate-vapid-keys)
2. Add keys to .env file
3. Run: docker-compose exec app npx prisma db push
4. Rebuild: docker-compose up --build

The web-push package is automatically installed during docker build.
All notification functionality works the same in Docker as locally.

docker-compose.yml

@@ -13,6 +13,10 @@ services:
       - NEXTAUTH_SECRET=${NEXTAUTH_SECRET}
       - AUTH_SECRET=${AUTH_SECRET}
       - AUTH_TRUST_HOST=true
+      # Push Notifications (VAPID)
+      - VAPID_PUBLIC_KEY=${VAPID_PUBLIC_KEY}
+      - VAPID_PRIVATE_KEY=${VAPID_PRIVATE_KEY}
+      - VAPID_SUBJECT=${VAPID_SUBJECT}
     restart: unless-stopped
     depends_on:
       - db-check