Payments: Apple IAP for webinar/class purchases (programCheckout flag)

[teetangh]

Context

Webinars and classes are one-to-few/one-to-many digital services — App Review Guideline 3.1.3(d)'s external-payment exemption does NOT apply; iOS purchases must go through Apple In-App Purchase. The MVP keeps program browsing read-only and gates purchase CTAs behind FeatureFlags.programCheckout (false).

Scope

1. StoreKit 2 integration (in_app_purchase plugin) for webinar/class enrollment on iOS; Razorpay/Stripe acceptable on Android
2. Server-side receipt validation endpoint + mapping to Payment (gateway enum needs an APPLE_IAP-style value — coordinate schema change with the web repo, which owns migrations; see chore: Consolidate Prisma schema management across web and mobile backends #18)
3. Pricing parity strategy: Apple's 15–30% commission vs web pricing
4. Flip FeatureFlags.programCheckout

Notes

• Until then, ComingSoonScreen placeholders cover program purchase entry points
• Waitlist enrollment ties into this — see Checkout: Add webinar/class enrollment and waitlist system #46

Related: #46, #113