feat: Implement alert creation service with FCM notifications

- Add AlertCreationService to handle alert creation and notifications.
- Implement idempotency key support to prevent duplicate alerts.
- Create alert notification service for sending FCM messages.
- Add tests for alert creation and notification services.
- Document FCM notification flow and alert creation process.

.env.example

@@ -5,6 +5,22 @@ DB_PASSWORD=your_secure_password_here
 API_KEY_SALT=generate_random_salt_with_openssl
 CORS_ORIGIN=http://localhost:3001
 
+# Backend runtime (used when starting backend directly, outside Docker Compose)
+PORT=3000
+DATABASE_URL=postgresql://transit_admin:your_secure_password_here@localhost:5432/polotsk_transit
+REDIS_URL=redis://localhost:6379
+JWT_SECRET=replace_with_a_long_random_string
+
+# Firebase Cloud Messaging (server-side alert notifications)
+# Keep disabled to run without Firebase credentials.
+FCM_ENABLED=false
+FCM_TOPIC=route_alerts
+FCM_ANDROID_CHANNEL_ID=route_alerts
+FIREBASE_PROJECT_ID=
+# Standard Application Default Credentials path. Store the JSON file outside
+# the repository and leave this empty while FCM_ENABLED=false.
+GOOGLE_APPLICATION_CREDENTIALS=
+
 # Bootstrap admin
 BOOTSTRAP_ADMIN=true
 BOOTSTRAP_ADMIN_RESET_PASSWORD=false