security: Address critical security vulnerabilities and improve code quality

🔒 Security Fixes:
- Make webhook signature validation required for production
- Prevent information disclosure by returning 200 for all webhook responses
- Sanitize external error messages while preserving internal logging

🔧 Code Quality Improvements:
- Add URL validation to prevent localhost usage in production
- Create currency utilities for proper handling of non-centesimal currencies
- Replace unsafe 'any' types with type-safe ProviderData wrapper
- Add comprehensive input validation for amounts, currencies, and descriptions
- Set default Stripe API version for consistency

📦 New Features:
- Currency conversion utilities supporting JPY, KRW, and other special cases
- Type-safe provider data structure with metadata
- Enhanced validation functions for payment data

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/providers/index.ts

@@ -1,3 +1,4 @@
 export * from './mollie'
 export * from './stripe'
 export * from './types'
+export * from './currency'