feat: add User registration application layer

[zigzagdev]

Motivation

With the infrastructure layer in place, the application layer is needed to orchestrate user creation. This PR introduces the command/use-case structure that sits between the presentation layer and the repository, providing input validation and a clear entry point for the create-user flow.

What I have done

• Added CreateUserCommand — validates that all required keys (first_name, last_name, email, age_range, subscription_tier) are present in the incoming array; exposes data via public readonly properties
• Added CreateUserUseCase — receives a CreateUserCommand and delegates to UserRepositoryInterface, returning a UserDto
• Added UserDtoFactory — follows the existing factory pattern under QueryUseCases/Factory/Dto/; builds a UserDto from a raw attribute array
• Updated UserDto — replaced the empty stub with typed public readonly properties
• Updated UserRepositoryInterface and UserRepository — changed the signature from UserEntity to CreateUserCommand; the repository now converts the command to DB columns directly and uses UserDtoFactory to build the return value
• Updated UserRepositoryTest — aligned with the new CreateUserCommand-based signature; mocks toArray() on the inserted model to feed UserDtoFactory

Test Results

PASS  Tests\Unit\Packages\Domains\UserRepositoryTest
✓ create user returns user dto on success
✓ create user throws exception when insert fails

Tests: 2 passed (3 assertions)

[zigzagdev]

Ok