Implement monetization features with RevenueCat integration, Paystack webhooks, and annual SKUs

[Copilot]

This PR implements comprehensive monetization features for Quicknote Pro by adding RevenueCat integration for subscription management, Cloudflare Worker for Paystack webhook processing, and expanded product offerings with annual subscription tiers.

Key Changes

🆕 Product SKUs and Pricing Updates

• Added annual subscription SKUs for Premium and Pro tiers:
  • quicknote_premium_annual - $19.99/year (17% savings)
  • quicknote_pro_annual - $29.99/year (17% savings)
• Updated pricing structure to match documentation:
  • Premium: $1.99/month, $19.99/year, $74.99 lifetime
  • Pro: $2.99/month, $29.99/year, $114.99 lifetime
  • Enterprise: $2.00/user/month, $20.00/user/year
• Centralized product management in lib/constants/product_ids.dart

🔗 RevenueCat Integration

• New RevenueCat service (lib/services/monetization/revenuecat_service.dart) for cross-platform subscription management
• Tiered entitlement system with feature-based access control
• Integrated with existing monetization service for seamless subscription status checking
• Purchase and restore functionality with analytics tracking

☁️ Cloudflare Worker for Paystack Webhooks

• Serverless webhook processor (cloudflare-worker/paystack-webhook.js) for handling Paystack payment events
• Secure signature verification and event processing
• RevenueCat synchronization to maintain consistent subscription state
• Production-ready deployment configuration with environment management

📚 Comprehensive Documentation

• Payment Setup Guide (docs/payment-setup-guide.md) with step-by-step instructions for:
  • Paystack account configuration
  • RevenueCat project setup
  • Cloudflare Worker deployment
  • Mobile app integration (iOS/Android)
  • Web integration with testing scenarios
• Deployment guide for Cloudflare Workers with security best practices
• Updated implementation guide with RevenueCat integration examples

🧪 Testing and Quality Assurance

• Comprehensive test suite for RevenueCat service functionality
• Product validation tests ensuring consistency between SKUs and pricing
• Feature access verification across different subscription tiers
• Updated documentation in README with payment setup information

Integration Points

The implementation seamlessly integrates with the existing monetization system:

// Check subscription status via RevenueCat
final tier = revenueCatService.getCurrentTier();
final hasFeature = revenueCatService.hasFeatureAccess('premium_features');

// Purchase annual subscription
await revenueCatService.purchaseProduct('quicknote_premium_annual');

Deployment Requirements

1. RevenueCat account with configured products matching the new SKUs
2. Paystack account for payment processing in African markets
3. Cloudflare Workers deployment for webhook processing
4. Environment variables for API keys and webhook secrets

Backward Compatibility

• All existing product IDs remain functional
• No breaking changes to the monetization service API
• Graceful fallback when RevenueCat is not configured
• Maintains existing feature gating behavior

This implementation provides a robust foundation for Quicknote Pro's monetization strategy with support for multiple payment providers, comprehensive webhook handling, and flexible subscription management across all platforms.

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