My Expo (React Native) queue app is live and fully functional with a FastAPI + MongoDB back-end; the only missing piece is push notifications. The flow is straightforward: • When a customer joins the queue—whether from the web front-end or the mobile app—the assigned barber must receive a push instantly. • When the barber taps “DONE,” the next person in line should see “You are next, please come” on their Android device. I already have a Firebase project set up, so please use Firebase Cloud Messaging rather than OneSignal. No UI redesign or business-logic tweaks are needed; the task is limited to notification plumbing and reliability. What I expect you to handle end-to-end: 1. Integrate the FCM SDK into the existing Expo codebase and ensure it compiles into a standalone Android APK (not just the Expo preview). 2. Register and store device tokens securely, tied to user roles (barber vs. customer) in MongoDB. 3. Add the FastAPI endpoints or service modules that fire FCM messages at the two trigger points above. 4. Verify delivery on real hardware with a signed APK and share APKs or test builds so I can confirm. Acceptance is simple: both push events must arrive consistently within a couple of seconds on every test run, and tokens must refresh gracefully after reinstalls or updates. Once those boxes are ticked, we are done.