Authoritative status check for a scan PaymentTransaction.

1. Returns immediately when the row already has a terminal status (the webhook beat us here).
2. Otherwise calls the provider's verify endpoint and updates the row in place.
3. Re-publishes the SSE event-bus snapshot so any reconnecting listener sees the result.

Race-safe: webhook arrival between the row read and the provider call is handled — the
next read will see the terminal row.