Add PayloadCMS type definitions, Prettier config, and PNPM lockfile

CLAUDE.md

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+# PayloadCMS Billing Plugin Development Guidelines
+
+## Project Overview
+
+This is a PayloadCMS plugin that provides billing and payment functionality with multiple payment provider integrations (Stripe, Mollie) and a test payment provider for local development.
+
+## Architecture Principles
+
+### Core Design
+- **Provider Abstraction**: All payment providers implement a common interface for consistency
+- **TypeScript First**: Full TypeScript support with strict typing throughout
+- **PayloadCMS Integration**: Deep integration with Payload collections, hooks, and admin UI
+- **Extensible**: Easy to add new payment providers through the common interface
+- **Developer Experience**: Comprehensive testing tools and local development support
+
+### Payment Provider Interface
+All payment providers must implement the `PaymentProvider` interface:
+```typescript
+interface PaymentProvider {
+  createPayment(options: CreatePaymentOptions): Promise<Payment>
+  retrievePayment(id: string): Promise<Payment>
+  cancelPayment(id: string): Promise<Payment>
+  refundPayment(id: string, amount?: number): Promise<Refund>
+  handleWebhook(request: Request, signature?: string): Promise<WebhookEvent>
+}
+```
+
+### Collections Structure
+- **Payments**: Core payment tracking with provider-specific data
+- **Customers**: Customer management with billing information
+- **Invoices**: Invoice generation and management
+- **Refunds**: Refund tracking and management
+
+## Code Organization
+
+```
+src/
+├── providers/           # Payment provider implementations
+│   ├── stripe/         # Stripe integration
+│   ├── mollie/         # Mollie integration
+│   ├── test/           # Test provider for development
+│   └── base/           # Base provider interface and utilities
+├── collections/        # PayloadCMS collection configurations
+├── endpoints/          # API endpoints (webhooks, etc.)
+├── hooks/             # PayloadCMS lifecycle hooks
+├── admin/             # Admin UI components and extensions
+├── types/             # TypeScript type definitions
+└── utils/             # Shared utilities and helpers
+```
+
+## Development Guidelines
+
+### Payment Provider Development
+1. **Implement Base Interface**: All providers must implement `PaymentProvider`
+2. **Error Handling**: Use consistent error types and proper error propagation
+3. **Webhook Security**: Always verify webhook signatures and implement replay protection
+4. **Idempotency**: Support idempotent operations where possible
+5. **Logging**: Use structured logging for debugging and monitoring
+
+### Testing Strategy
+- **Unit Tests**: Test individual provider methods and utilities
+- **Integration Tests**: Test provider integrations with mock APIs
+- **E2E Tests**: Test complete payment flows using test provider
+- **Webhook Tests**: Test webhook handling with various scenarios
+
+### TypeScript Guidelines
+- Use strict TypeScript configuration
+- Define proper interfaces for all external API responses
+- Use discriminated unions for provider-specific data
+- Implement proper generic types for extensibility
+
+### PayloadCMS Integration
+- Follow PayloadCMS plugin patterns and conventions
+- Use proper collection configurations with access control
+- Implement admin UI components using PayloadCMS patterns
+- Utilize PayloadCMS hooks for business logic
+
+### Security Considerations
+- **Webhook Verification**: Always verify webhook signatures
+- **API Key Storage**: Use environment variables for sensitive data
+- **Access Control**: Implement proper PayloadCMS access control
+- **Input Validation**: Validate all inputs and sanitize data
+- **Audit Logging**: Log all payment operations for audit trails
+
+## Environment Configuration
+
+### Required Environment Variables
+```bash
+# Stripe Configuration
+STRIPE_SECRET_KEY=sk_test_...
+STRIPE_PUBLISHABLE_KEY=pk_test_...
+STRIPE_WEBHOOK_SECRET=whsec_...
+
+# Mollie Configuration  
+MOLLIE_API_KEY=test_...
+MOLLIE_WEBHOOK_URL=https://yourapp.com/api/billing/webhooks/mollie
+
+# Test Provider Configuration
+NODE_ENV=development # Enables test provider
+```
+
+### Development Setup
+1. Use test provider for local development
+2. Configure webhook forwarding tools (ngrok, etc.) for local webhook testing
+3. Use provider sandbox/test modes during development
+4. Implement comprehensive logging for debugging
+
+## Plugin Configuration
+
+### Basic Configuration
+```typescript
+billingPlugin({
+  providers: {
+    // Provider configurations
+  },
+  collections: {
+    // Collection name overrides
+  },
+  webhooks: {
+    // Webhook configuration
+  },
+  admin: {
+    // Admin UI configuration
+  }
+})
+```
+
+### Advanced Configuration
+- Custom collection schemas
+- Provider-specific options
+- Webhook endpoint customization
+- Admin UI customization
+
+## Error Handling Strategy
+
+### Provider Errors
+- Map provider-specific errors to common error types
+- Preserve original error information for debugging
+- Implement proper retry logic for transient failures
+
+### Webhook Errors
+- Handle duplicate webhooks gracefully
+- Implement proper error responses for webhook failures
+- Log webhook processing errors with context
+
+## Performance Considerations
+- Implement proper caching where appropriate
+- Use database indexes for payment queries
+- Optimize webhook processing for high throughput
+- Consider rate limiting for API endpoints
+
+## Monitoring and Observability
+- Log all payment operations with structured data
+- Track payment success/failure rates
+- Monitor webhook processing times
+- Implement health check endpoints
+
+## Documentation Requirements
+- Document all public APIs with examples
+- Provide integration guides for each payment provider
+- Include troubleshooting guides for common issues
+- Maintain up-to-date TypeScript documentation