[Rishiii57]

Hey team,

This implements ratability locking for rated contests as requested in #2463.

What was happening

When someone is retroactively disqualified from a contest, all future rated contests get re-rated. During re-rating, rate_contest() previously re-evaluated each user's eligibility against rating_floor and rating_ceiling dynamically based on their recalculated last_rating.

If a past disqualification shifted a user's rating across the floor or ceiling boundary, participants could suddenly flip between rated and unrated after the contest was long over. For example, someone who joined a Div. 2 contest expecting to be unrated could suddenly get rated, or a rated participant could unexpectedly lose their contest rating.

What changed

1. Added a ratable_users ManyToManyField on Contest to track which users are eligible to be rated.
2. Added a helper method contest.is_ratable_for(profile) that checks whether the contest is rated, verifies the user is not in rate_exclude, and checks that their current rating falls between rating_floor and rating_ceiling (defaulting newcomers to 1200 / RATING_INIT).
3. In ContestJoin.join_contest(), when a user enters as a live participant, if contest.is_ratable_for(profile) is True, they are added to contest.ratable_users. This locks their ratability at the time they actually enter.
4. In rate_contest(), if contest.ratable_users.exists() is True, it filters participants by ratable_users.all(). If ratable_users is empty (such as older legacy contests), it falls back to the previous dynamic floor/ceiling check for complete backward compatibility.
5. Added ratable_users to the Django admin contest form with filter_horizontal and readonly lock enforcement.
6. Added migration 0153_contest_ratable_users.py.

Testing

• Verified with a standalone reproducer (reproduce_dmoj_2463.py) demonstrating how retroactive rating shifts flip user status on unpatched code and stay completely stable on patched code.
• Added RatabilityLockingTestCase in judge/models/tests/test_contest.py covering boundary checks, newcomer defaults, exclude lists, and queryset logic.
• Passed all 24 native unit tests in manage.py test with 0 failures.
• Passed flake8 with 0 warnings or errors.