Bug: Race Condition in Public Submission Toggling Allows Multiple Public Submissions

## Description
When is_restricted_to_select_one_submission is enabled for a challenge phase, multiple submissions can become public simultaneously due to a race condition. This violates the business rule that only one submission should be public at a time.

## Location
**File:** apps/jobs/views.py
**Lines:**
- 342-349 (in challenge_submission function)
- 475-485 (in change_submission_data_and_visibility function)

## Reproduction
1. Save the attached test script as test_bug_race_condition.py
2. Run: python3 test_bug_race_condition.py
3. Observe the output showing 2 public submissions instead of 1

**Test Output:**
Initial: 1 public submission(s) Final: 2 public submission(s) - IDs: [2, 3]

## Expected Behavior
When a challenge phase has is_restricted_to_select_one_submission=True:
- Only 1 submission should be public at any given time
- When making a new submission public, the existing public submission should be made private first
- This should work correctly even with concurrent requests

## Actual Behavior
- Multiple submissions can become public simultaneously
- The race condition occurs because there is no database-level locking
- Both concurrent requests pass the count() == 1 check at the same time
- Both proceed to make their submissions public


Start in apps/jobs/views.py at lines 342-349 in challenge_submission and 475-485 in change_submission_data_and_visibility. Run the supplied test_bug_race_condition.py reproduction and inspect how concurrent requests pass the count check without database-level locking. Done means concurrent requests leave only one public submission for a restricted challenge phase, with regression coverage for the reproduced case.

Written by the indexing model from the issue text.