feat: add RevenueCat integration templates and modular scaffolding support for native apps

[Alisha-21-cloud]

Add RevenueCat Payments Integration for Native Apps

Description

This PR introduces RevenueCat as a new payments provider for in-app purchases on native (Expo / React Native) applications.

Unlike the existing Polar integration (which is web-focused and Better-Auth dependent), RevenueCat is native-gated and auth-agnostic. It is exclusively offered when a native frontend is selected (native-bare, native-uniwind, or native-unistyles) and when a backend is present.

Key Features & Implementation Details

1. Core Integration & Configuration

• Schema & Validation: Added revenuecat to PaymentsSchema.
  • Prompts conditionally offer RevenueCat only when a native frontend is present.
  • validatePaymentsCompatibility now correctly errors if RevenueCat is selected without a native frontend.
  • Added new Matrix oracle cases (e.g., payments-revenuecat-requires-native-frontend).
• Web Stack-Builder: Added RevenueCat to apps/web with a native-frontend requirement constraint, mirroring how Polar gates on Better-Auth.

2. Convex Backend Support

• Component Wiring: Integrates the convex-revenuecat component via app.use(revenuecat) in convex.config.ts.
• Webhook Routing: Registers a RevenueCat webhook route (POST /webhooks/revenuecat).
  • Template Handler Logic: If Better-Auth is present, the webhook is appended to the existing Better-Auth http.ts. If no Better-Auth is used (e.g., Clerk or none), a standalone http.ts is generated to avoid clobbering routes.
• Client API: Exposes a revenuecat.ts client with utilities like hasEntitlement, isSubscriber, and getActiveSubscriptions.
• Other Backends: For backends other than Convex, the implementation gracefully falls back to a pure frontend React Native SDK integration (react-native-purchases), as RevenueCat handles subscription state independently.

3. Native Scaffolding & Components

• Provider & SDK: Scaffolded a shared, auth-agnostic RevenueCatProvider and a lib/revenue-cat SDK layer.
• UI Components: Added subscription-status-card and paywall-example components.
  • Fully styled across all three variants: standard StyleSheet (native-bare), NativeWind (native-uniwind), and unistyles (native-unistyles).
• Layout Integration: The RevenueCatProvider is conditionally wrapped and mounted within each variant's app/_layout.tsx.

4. Environment Variables

• Native (.env): Scaffolds EXPO_PUBLIC_REVENUECAT_IOS_KEY, EXPO_PUBLIC_REVENUECAT_ANDROID_KEY, and EXPO_PUBLIC_REVENUECAT_ENTITLEMENT_ID.
• Convex (.env.local): Adds REVENUECAT_WEBHOOK_AUTH along with helpful setup comments for local development.

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