feat: implement Presentation layer for User Delete API / Presentation層のdeleteUser実装

[zigzagdev]

Parent Issue / 親Issue

#514

Motivation / 目的

Implement the Controller and route for the User Delete API as part of #514.

ユーザー削除APIのPresentation層（#514）として、Controllerとエンドポイントを実装します。

What to do / 実施内容

• UserController::deleteUser(): call DeleteUserUseCase
  • 200 on success
  • 404 when user not found
  • 500 on other errors with Log::error
• Route: DELETE /api/v1/users/{id}

Tests / テスト

• test_delete_user_returns_200_when_user_exists
• test_delete_user_returns_404_when_user_not_found