feat: implement Application layer for User Delete API / Application層のDeleteUserUseCase実装

[zigzagdev]

Parent Issue / 親Issue

#514

Motivation / 目的

Implement the Application layer for the User Delete API as part of #514.

ユーザー削除APIのApplication層（#514）として、DeleteUserUseCase を実装します。

What to do / 実施内容

• DeleteUserUseCase: call UserRepositroyInterface::deleteUser() and return void
  • Throw Exception if user not found (propagated from repository)

Tests / テスト

• test_handle_deletes_user_when_exists
• test_handle_throws_exception_when_user_not_found