[Task] oauth 2.0

### ISSUE
- Group: `server`
- Type: `feature`
- Detail: oauth 2.0 을 통한 외부 인증 구현

### TODO
1. [x] 액세스 토큰 받기
2. [x] 토큰 교환
3. [x] 토큰 상태에 따른 상태 및 데이터 전송

### Estimated time
> Pick one

`1h`

### Labels
- Estimated time: `E: 1h`
- Group : `server`
- Sprint: `Sprint__NUMBER__1`
- Urgency: `Middle`

The issue names no files, tests, OAuth provider, or acceptance criteria. Start by locating the server's authentication entry point and determine what remains beyond the checked token tasks; the definition of done must specify the external-auth flow and its tests.

Written by the indexing model from the issue text.