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.

PUSH_NOTIFICATIONS_DOCKER_CHECKLIST.md

@@ -0,0 +1,81 @@
+# Push Notifications Docker Setup Checklist
+
+## ✅ What's Already Done
+
+- [x] Code implementation complete
+- [x] Docker configuration updated
+- [x] Environment variables added to docker-compose.yml
+- [x] Dependencies added to package.json (will install on build)
+
+## 🔧 What You Need to Do
+
+### [ ] 1. Generate VAPID Keys
+
+Run this command:
+```bash
+npx web-push generate-vapid-keys
+```
+
+Or if you don't have Node locally:
+```bash
+docker run --rm -it node:20-slim sh -c "npm install -g web-push && web-push generate-vapid-keys"
+```
+
+### [ ] 2. Add Keys to .env
+
+Copy the output and add to your `.env` file:
+```env
+VAPID_PUBLIC_KEY=BNxxx...
+VAPID_PRIVATE_KEY=xxx...
+VAPID_SUBJECT=mailto:your-email@example.com
+```
+
+### [ ] 3. Update Database
+
+Start containers and run migration:
+```bash
+docker-compose up -d
+docker-compose exec app npx prisma db push
+```
+
+### [ ] 4. Rebuild Containers
+
+```bash
+docker-compose down
+docker-compose up --build
+```
+
+## ✅ Verify It Works
+
+1. Open http://localhost:3500
+2. Log in
+3. Wait 3 seconds
+4. See notification prompt appear
+5. Click "Enable Notifications"
+6. Test by adding items from another browser
+
+## 🎉 Done!
+
+Once you complete these 4 steps, push notifications will work automatically!
+
+## Quick Reference
+
+**Check if VAPID keys are loaded:**
+```bash
+docker-compose exec app env | grep VAPID
+```
+
+**Check database tables:**
+```bash
+docker-compose exec app npx prisma studio
+```
+
+**View logs:**
+```bash
+docker-compose logs -f app
+```
+
+**Restart after .env changes:**
+```bash
+docker-compose restart
+```