Cross-Platform Subscriptions & API Update

I need the subscription flow in my existing mobile app switched from dummy buttons to real, production-ready in-app purchases on both iOS and Android. The build already compiles and runs; what is missing is the full StoreKit / Google Play Billing integration, receipt validation, restore logic, and the UI state changes that follow an active or expired plan.

While you are inside the codebase, I also want to point the app to a brand-new backend. The new API is live, but several endpoints, parameters, and response shapes differ from the previous version. I will supply the OpenAPI file and a changelog. Your job is to swap the base URL, update the affected service calls, and confirm that every screen still loads its data correctly.

Deliverables
• Seamless subscription-based in-app purchase flow on iOS & Android (including restore and cancel handling)
• Code updated to consume the new API, with all impacted endpoints working as before
• A short test report or screen recording that demonstrates happy-path and failure-path behaviour for both tasks
• Clean, well-commented commits or a pull request so I can review the changes quickly

Acceptance criteria
The app must pass review on TestFlight and Google Play’s internal testing with no purchase or network errors, and all modified endpoints must return the expected data without crashes or console warnings.

If you have shipped cross-platform subscriptions before and are comfortable refactoring API layers, I would love to get this wrapped up soon.