Fix deadline check

[S1ro1]

No description provided.

[Copilot]

Pull request overview

This PR fixes a timezone-related bug in the deadline check functionality by making the current time timezone-aware (UTC). The change ensures correct comparison between the current time and database-stored deadlines, which are timezone-aware datetime objects returned by PostgreSQL.

• Changed import from from datetime import datetime to import datetime for better namespace clarity
• Updated check_deadline() to use datetime.datetime.now(datetime.timezone.utc) instead of naive datetime.now()

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[Copilot]

The tests in test_check_deadline() use naive datetime objects (datetime.datetime.now() without timezone), but the production code now uses timezone-aware datetime objects (datetime.datetime.now(datetime.timezone.utc)). This mismatch means the tests are not accurately testing the actual behavior.

The tests should be updated to use timezone-aware datetime objects to match the production code:

def test_check_deadline():
    # Test valid deadline (future)
    future_deadline: LeaderboardItem = {
        "deadline": datetime.datetime.now(datetime.timezone.utc) + datetime.timedelta(days=1),
        "name": "test",
    }
    submission.check_deadline(future_deadline)  # Should not raise

    # Test expired deadline
    past_deadline: LeaderboardItem = {
        "deadline": datetime.datetime.now(datetime.timezone.utc) - datetime.timedelta(days=1),
        "name": "test",
    }
    # ...

Additionally, the mock in mock_backend fixture (line 30) should also use timezone-aware datetime.