Migrate contest, problem authors/curators/testers to Role

[whlongg]

Description

Refactor role management for Contest by migrating the separate authors, curators, and testers M2M relationships to a unified ContestRole model.

Type of change: refactor

What

• add shared role constants and the new ContestRole model
• add a single migration that creates the role table, copies existing data, and removes the legacy M2M fields
• update contest permission and visibility logic to use the new role table
• keep compatibility at the model API level by exposing authors, curators, and testers via query adapters, so most callers continue to work unchanged
• update admin, queryset helpers, submission visibility logic, and related tests/utilities to use the new role structure
• add unit tests for contest role behavior

Why

The existing role system stores authors, curators, and testers as separate M2M fields on Contest, which duplicates the same concept in multiple places and makes permission logic harder to maintain. Consolidating these relationships into a dedicated role table gives us a single extensible structure while preserving current behavior.

A follow-up PR will apply the same pattern to Problem.

Fixes #563 (partial — contest side)

How Has This Been Tested?

• python manage.py migrate judge
• python manage.py test judge.models.tests.test_contest judge.models.tests.test_role --keepdb
• python manage.py test judge.views.tests.test_submission --keepdb
• flake8 judge/models/role.py judge/models/__init__.py judge/models/contest.py judge/admin/contest.py judge/views/contests.py judge/views/organization.py judge/views/submission.py judge/views/api/api_v2.py judge/jinja2/submission.py judge/models/tests/test_contest.py judge/models/tests/test_role.py judge/models/tests/util.py judge/migrations/0220_migrate_contest_roles.py

Checklist

• I have explained the purpose of this PR.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the README/documentation
• Any dependent changes have been merged and published in downstream modules
• Informed of breaking changes, testing and migrations (if applicable).
• Attached screenshots (if applicable).

Notes for reviewers

• this PR includes a data migration; please run migrations in order through the full Django migration chain
• old authors / curators / testers M2M fields on Contest are removed, but compatibility is preserved at the model usage level via adapter properties
• Problem role migration will follow in a separate PR
• no UI screenshots are included because this change is backend/model/admin focused

By submitting this pull request, I confirm that my contribution is made under the terms of the AGPL-3.0 License.