Revolut Terminal

- SECURITY: Order creation and the terminal push now send a Revolut-Request-Id idempotency key (bounded hash; from an explicit per-attempt id for the order, from the order id for the push) so a retried/timed-out request no longer creates a duplicate payable order or pushes a second payment to the reader.
- Partial refund aborts if the order-currency lookup fails instead of silently falling through to a FULL refund with the amount dropped.
- Refund status is mapped from the returned refund state instead of hardcoding 'refunded'.
- Minor-unit conversion rounds (ROUND_HALF_UP) instead of truncating, and status/amount readback uses the shared currency-exponent helper (was an incomplete JPY/KRW-only list).

- Initial release with Revolut Terminal cloud push payments
- Server-to-server REST API integration (no SDK required)
- Support for Revolut Terminal and Revolut Reader hardware
- Terminal discovery via Merchant API
- Cloud-based payment push to terminals
- Payment status polling
- Payment cancellation support
- Full and partial refund support