Function retire_capped_mails 

pub async fn retire_capped_mails(
    conn: &mut PgConnection,
    actor_user_id: Uuid,
    actor_role: &str,
    course_id: Uuid,
    student_number: &str,
    reason: &str,
) -> ModelResult<i64>

Retires the linking-mail rows the caps are counting for this person, so the ordinary claim path can take a slot again. No parameter relaxes a cap: the single writer of the ledger evaluates them from the rows that exist, so getting past one means soft-deleting rows, audited as its own action.