Processes a refund for a previously charged application. This operation reverses the charge and updates the application status. Accepted application statuses: APPROVED or FUNDED. Refund window: refunds are accepted only within 20 days of the application signing date (signed_at). Requests outside this window are rejected with 409 CONFLICT. Amount validation: if the requested amount exceeds the remaining refundable balance (principal minus all prior refunds), the request is rejected with 409 CONFLICT. The balance is never silently reduced — the partner must request a corrected amount. pix_key and pix_key_type are optional. If not provided and a PIX transfer to the customer is needed (transfer_amount > 0), the refund transitions to ACTION_REQUIRED. Use PUT /application/{id}/refund/{refund_id} to supply the PIX key later. refund_type is optional. If omitted, it is deduced automatically: FULL when the cumulative refunds cover the full principal, PARTIAL otherwise. Credit limit: released immediately when the refund is created — a refund.limit_restored webhook is dispatched at this point regardless of refund status or PIX key. Status flow: PENDING → COMPLETED (no transfer needed) | ACTION_REQUIRED (transfer needed, no PIX key) | LIMIT_RESTORED (transfer needed, PIX key provided)