Function apply_outcome 

pub(crate) async fn apply_outcome(
    conn: &mut PgConnection,
    registration: &CreditRegistration,
    outcome: &Outcome,
    event: OutcomeEvent<'_>,
    expected_from_state: Option<CreditRegistrationState>,
) -> Result<()>

Applies one decided outcome to one row, with the exchange that produced it.

expected_from_state guards against writing back a decision made from a row snapshot that an await (an external call, or just the gap since claiming) has let go stale: pass the state the phase itself put the row in before that await, or Some(registration.state) when the phase never moved the row before its own await.