feat: implement Presentation layer for User findById API / findById APIのPresentation層実装

[zigzagdev]

Overview / 概要

Implement the Controller and route for the User findById API, as part of #507.

#507 の一環として、ユーザー取得APIのControllerとRouteを実装する。

Requirements / 要件

• Controller: UserController::show() with try/catch
  • 200 on success, 404 when not found, 500 on server error with Log
  • Use Request for the incoming request
• ViewModel: Reuse existing UserViewModel and UserViewModelFactory
• Route: GET /api/v1/user/{id}

Tasks / タスク

• Add show action to UserController
• Register route GET /api/v1/user/{id} in api.php
• Write unit tests for Controller layer to meet Codecov patch coverage target

Parent issue / 親Issue

#507