Plan In-App Payments & Monetization

Background: Native [iOS / Android / both] app. Monetization model:
[one-time purchase / subscription / consumables / freemium]. What's paid:
[describe the gated features or content].

Use case: Plan the in-app purchase implementation and the surrounding
entitlement logic.

Implementation details:
- iOS: StoreKit 2 (products, purchase, transaction verification,
  restore). Android: Google Play Billing Library.
- When Apple/Google billing is mandatory vs when external payment is
  allowed—flag the store-policy traps
- Server-side receipt/entitlement validation vs on-device, and why
- A vendor abstraction (or RevenueCat-style service) to handle both
  stores—evaluate build-vs-buy here too
- Handling subscription states: active, grace period, billing retry,
  expired, refunded
- Restoring purchases and syncing entitlements across a user's devices

Layer & dependencies: A single "entitlements" source of truth the app
reads to unlock features; purchase plumbing stays in the data layer.

Deliverable: A purchase-flow design for my model, the entitlement state
machine, the validation approach with its tradeoffs, the store-policy
pitfalls to avoid, and a build-vs-buy call on a billing abstraction.