Dual-Platform Subscription Integration

I have an AI sports-results prediction app that is ready for public release, and now I need seamless in-app subscription handling on both Google Play and Apple App Store. The core product is already built; your task is to wire up the recurring billing so users can unlock the premium prediction feed inside the app.

What I need from you
• Set up in-app purchases and auto-renewable subscriptions in Google Play Console and App Store Connect.
• Implement the client-side purchase flow in my existing Flutter codebase (I’m using the latest stable build), handle upgrades, downgrades, and cancellations, and surface any edge-case errors cleanly to the user.
• Configure the server-side receipt validation and subscription status checks (Firebase Cloud Functions are in place, but I’m open to another lightweight Node or Python service if you prefer).
• Make sure pricing tiers, free-trial logic, and introductory offers are in sync across both stores.
• Supply clear, commented code plus a one-page handover guide so I can update products or prices later without diving back into the code.

Acceptance criteria
1. Test accounts on Android and iOS can subscribe, cancel, and resubscribe without manual intervention.
2. Premium content (the sports-prediction feed) locks/unlocks instantly according to the server-verified subscription status.
3. Store review guidelines for payments are fully met and the builds pass internal review in both consoles.
4. Code builds cleanly and all new logic is covered by unit tests or a Postman collection for server endpoints.

If you have shipped cross-platform in-app subscriptions before, especially for prediction or data-driven apps, I’d love to see a quick note on your preferred validation stack.