feat: implement deleteUser in Infra layer / Infra層のdeleteUser実装

[zigzagdev]

Parent Issue / 親Issue

#514

Motivation / 目的

Implement the repository layer for deleting user data as part of the User Delete API (#514).

ユーザー削除APIのInfra層（#514）として、ユーザーデータを削除するリポジトリ層を実装します。

What to do / 実施内容

• UserRepositroyInterface: add deleteUser(int $id): void
• UserRepository: implement deleteUser
  • Find user by ID via find(), throw Exception if not found
  • Call delete() to remove the record

Tests / テスト

• test_deleteUser_deletes_user_when_exists
• test_deleteUser_throws_exception_when_user_not_found