SumUp Terminal

- Amounts use the currency's actual minor unit (0 for zero-decimal currencies like JPY, else 2) with ROUND_HALF_UP, instead of a hardcoded minor_unit:2 and int(amount*100) that truncated and mis-scaled non-2-decimal currencies.
- SECURITY: Reader checkout sends a client_transaction_id from an explicit per-attempt id (when supplied) so a retried/timed-out request de-dups instead of pushing a second checkout to the reader.
- Refund amount is quantized instead of passed through float(), removing binary float precision error.

- Initial release with SumUp Cloud API terminal support
- Cloud-based payment initiation via Readers Checkout API
- Status polling for payment completion
- Reader pairing and discovery
- Checkout termination (cancel) support
- Full and partial refund support
- Support for Solo and Solo Lite readers