- Add AuthHandler struct with authService dependency and NewAuthHandler constructor
- Add LoginRequest/LoginResponse/RegisterRequest/ChangePasswordRequest structs with validation tags
- Add Login method with username/password validation, token generation, and user response
- Add Register method with email/password/role validation and user creation
- Add RefreshToken method with JWT refresh token validation
- Add error code constants for validation/auth/resource/system/business logic errors (VALIDATION_FAILED/UNAUTHORIZED/NOT_FOUND/INTERNAL_ERROR/etc)
- Add EnhancedErrorResponse with error/code/details/context/timestamp/request_id fields and ValidationErrorResponse with field-specific errors array
- Add handleError function with gorm.ErrRecordNotFound/ErrDuplicatedKey/ErrInvalidData handling