feat: implement Firebase ID token authentication for passenger telemetry and update related configurations

.env.example

@@ -39,6 +39,11 @@ ADMIN_EMAIL=admin@polotsk-transit.local
 # Frontend
 API_URL=/api/v1
 
+# Required by the passenger telemetry endpoint to verify Firebase ID tokens.
+# Use the same Firebase project as the mobile app; keep the service account JSON outside the repo.
+FIREBASE_PROJECT_ID=your-firebase-project-id
+GOOGLE_APPLICATION_CREDENTIALS=/absolute/private/path/firebase-service-account.json
+
 # Production settings (uncomment for production)
 # NODE_ENV=production
 # CORS_ORIGIN=https://admin.yourdomain.com