글 작성 시 유저 아이디 찾아서 같이 저장되는 로직

### ISSUE
- Group: `server`
- Type: `feature`
- Detail:

### TODO
- [ ] isAuthorized 완성
- [ ] 글 작성 시 유저 아이디를 같이 포스트

### Estimated time
>시간을 수정하세요
`3h`

### Labels
- Estimated time: `E: 3h`
- Group : `server`
- Sprint: Sprint5
- Urgency: `High`

Start in the server-side authorization and post-creation entry points, locating isAuthorized and the handler that creates a post. Confirm the intended user ID source and verify that authorization and saved ownership are covered by the relevant server tests.

Written by the indexing model from the issue text.