Implement complete subscription monetization system for repositories with Stripe and PayPal integration. Includes:
- Database models and migrations for monetization settings, subscription products, and user subscriptions
- Payment provider abstraction layer with Stripe and PayPal implementations
- Admin UI for configuring payment providers and viewing subscriptions
- Repository settings UI for managing subscription products and tiers
- Subscription checkout flow and webhook handlers for payment events
- Access control to gate repository code behind active subscriptions
