Handle PIS payment status polling and async settlement notification for open banking payment initiation across UK and EU

Verified steps

1. After initiating a payment, persist the PaymentId (UK) or paymentId (Berlin Group) and a correlation to your internal order or transaction ID; do not treat the 201 Created response as payment execution — it confirms only that the initiation instruction was accepted
2. For UK Open Banking: poll GET /domestic-payments/{DomesticPaymentId} to retrieve status; map UK statuses (Pending, AcceptedSettlementInProcess, AcceptedSettlementCompleted, Rejected) to your internal state machine
3. For PSD2/Berlin Group: poll GET /payments/{paymentId}/status; map Berlin Group status codes (ACCP, ACSC, RJCT, PDNG, CANC) — ACSC (AcceptedSettlementCompleted) is the terminal success state
4. Register webhooks wherever the ASPSP or aggregator supports them; UK Open Banking event notifications use the /event-subscriptions and /events endpoints — subscribe to urn:uk:org:openbanking:events:resource-update events for payment objects
5. Implement a polling fallback with exponential backoff for ASPSPs that do not support push notifications; start with a 10-second interval and back off to a maximum of 5-minute intervals; set a hard timeout (e.g., 2 hours) after which you mark the payment as timed-out and alert operations
6. Reconcile completed payments against your order management system using the EndToEndIdentification (UK) or endToEndIdentification (Berlin Group) reference included in the payment initiation — this field passes through to the beneficiary bank statement