Refactor Authentication Service and Implement User Verification & Redis Cache

[YanPetrov7]

• The authentication logic has been reorganized into its own dedicated directory for better structure and maintainability;
• Introduced MailService to handle the sending of user verification emails;
• Added the core logic for user verification, including generating and validating verification tokens;
• Added unit tests to ensure MailService integration with AuthController works as expected;
• Replaced the standard cache module with Redis to improve performance and scalability of the caching mechanism.