[Feat] Auth domain development

[Zerohertz]

What type of PR is this?

• feat
• fix
• refactor
• test
• infra
• ci/cd
• docs
• chore

What this PR does / why we need it

• /auth/login endpoint 개발 (Password & GitHub OAuth)
• user domain refactoring
• PyTest 시 SQLite에서 create_all이 원활이 수행되지 않던 현상 해결 (related: [Feat] Auth domain development #22)

Which issue(s) this PR fixes

Related #22
Fixes #25

Special notes for your reviewer

Does this PR introduce a user-facing change?

- `/auth/login` endpoint 개발 (Password & GitHub OAuth)
- `user` domain refactoring
- PyTest 시 SQLite에서 `create_all`이 원활이 수행되지 않던 현상 해결 (related: #22)

Additional documentation