Handle Adyen ACH Direct Debit returns and map NACHA return codes from chargeback webhooks

Steps

1. Accept the ACH Direct Debit payment normally, but treat the initial AUTHORISATION webhook as provisional, not final settlement — ACH debits can still bounce days later.
2. Listen for NOTIFICATION_OF_CHARGEBACK and CHARGEBACK webhook events — this is how Adyen models ACH returns, rather than a generic REFUND_FAILED event.
3. Read the chargebackReasonCode field on the chargeback webhook payload to get the underlying NACHA return code (for example R01, R02, R03) and branch retry or dunning logic accordingly.
4. Respect Adyen's built-in delay of a few days before processing merchant-initiated refunds on ACH payments — it exists specifically to avoid a race between a refund and a same-window chargeback.
5. Verify the exact JSON location of chargebackReasonCode against the live Adyen API Explorer/webhook schema before wiring field access in code, since static docs don't render the nesting unambiguously.