feat: add universal verification endpoint and M-Pesa support

• Introduce universal verification router (POST /verify) that automatically detects payment provider based on reference format
• Add M-Pesa verification endpoint with API integration and PDF parsing
• Enhance API key security by implementing hashed storage with backward compatibility
• Improve Dashen verification with retry mechanism and increase timeouts across services
• Update database schema with new ApiKey fields and User.createdAt
• Fix route precedence to prevent /verify from swallowing other endpoints
• Update documentation and Postman collection to reflect new features