I already have a credit-based auction app built with Flutter that talks to Firebase, but several core rules are still handled on the client. I want that logic shifted to the backend so everything is safer and easier to maintain. Here is exactly what I need: • Move the existing Authentication, Leaderboard and Auction logic from Flutter code into Firebase Cloud Functions / Firestore security rules. The app should call lightweight endpoints while all validation, credit checks and ranking updates happen server-side. • Add an “Instant Buy” flow inside the mobile app (our only sales channel). An admin can list a product with a quantity of 1 – 2 units and a fixed price. The first user who taps “buy” and completes payment gets it; the item must become instantly unavailable(expired) to everyone else. • Extend bidding rules so a user may offer more credits than currently in their wallet, but only if they settle the outstanding balance within 24 hours of winning. If they fail, the system must automatically revoke the win and pass the item to the next highest eligible bidder. • Implement a “Sell With Us” request form. The UI is already designed in Flutter; I just need the backend workflow: – When a user submits, the item enters a Pending state. – We (admins) can Approve or Decline inside the Firebase console or a simple web panel you provide. – On approval, notify the user and log that a shipping company will collect the product for us. I will review progress closely in on a Firebase, so please write clean, well-commented Cloud Functions and provide clear steps.