Function transition 

pub async fn transition(
    conn: &mut PgConnection,
    id: Uuid,
    transition: &Transition,
) -> ModelResult<CreditRegistration>

Moves a ledger row to a new state and appends the matching audit event, atomically.

Also stamps, so callers must not: state_entered_at (every call, including a self-transition), terminal_at (set on entry to a terminal state, cleared on leaving one), first_failed_at (the first failure only), registered_at, submitted_at, enrolment_checked_at (on leaving checking_enrolment), and clears enrolment_banner_dismissed_at on entry to no_usable_enrolment so a fresh enrolment problem shows the banner again.