feat(be): implement polygon collaborator api

[juheon429]

Description

문제 생성시 협업자를 관리하는 API를 구현하였습니다.

<구현 기능>

1. inviteCollaborator
   -협업자 초대는 해당 문제의 소유자, active editor 인 경우에만 가능
   -소유자가 초대시 active, active editor가 초대시 pending

2. getActive/PendingCollaborator
   -협업자 목록 반환, 요청 대기자 목록 반환

3. approve/rejectInvite
   -요청에 대한 수락 여부 -> 해당 문제 소유자만 가능

4. updateCollaboratorRole
   -협업자의 role 변경 -> 해당 문제 소유자만 가능

5. removeCollaborator
   -협업자 제거 -> 해당 문제 소유자만 가능

6. requestCollaboration
   -임의의 사용자가 협업 요청

Additional context

---

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines
• Read the Contributing Guidelines and follow the Commit Convention
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

Summary by CodeRabbit

• New Features

  • Added polygon collaboration management.
  • Users can invite, approve, reject, update, remove, and request collaborators.
  • Added active and pending collaborator listings.
  • Added role validation and permission checks for collaboration actions.

• Changes

  • Renamed the collaborator role “Viewer” to “Reviewer.”

[gemini-code-assist]

Code Review

This pull request introduces a new collaborator management system for polygon problems, including GraphQL resolvers and service methods for inviting, approving, rejecting, updating roles, and removing collaborators. The review identified potential security and logic issues: the Owner role should not be assignable via the API to prevent privilege escalation, and the getCollaboratorsByStatus method requires access control to prevent unauthorized users from viewing collaborator lists.

[zero1177]

일단 쭉 보면서 리뷰 달았어요!