[server] getRandomPost method 추가

> ### ISSUE
> * Group: `server`
> * Type: `feature`
> * Detail: 메인 페이지에 표시할 임의의 게시글 5개를 요청하는 함수 구현
>
> ### TODO
> 1. [x] 랜덤으로 게시글 5개 출력
>
> ### Estimated time
>
> ### `0.5h`
>
> ### Labels
> * Estimated time: `E: 0.5h`
> * Group : `server`
> * Sprint: `Sprint__2__`
> * Urgency: `Middle`

The issue provides no file, test, or entry point. Start by locating the server-side getRandomPost method, then verify how the main page requests posts and confirm that the completed behavior returns five randomly selected posts.

Written by the indexing model from the issue text.