Function lock_and_verify_named_uploads 

async fn lock_and_verify_named_uploads(
    tx: &mut Transaction<'_, Postgres>,
    exercise_id: Uuid,
    user_id: Uuid,
    submission: &StudentExerciseSlideSubmission,
) -> Result<(), ControllerError>

Re-checks the named uploads under the reaper’s row lock, inside the transaction that records the submission and before anything is written to it.