加了相应的unique constraint，并且使用原子的update_or_create，避免在数据库中产生两个相同的记录

Remember to uncomment return in 2025 Fall

Now accepts year + semester pair on command line.
Does the sorting in the SQL query, allowing O(n) counting invalid records.
Updates course priority in batch and added transaction.atomic

This reverts commit 186301c.

This is because the race condition resolution commit is now in another
PR.

Organize imports and add trailing newline