Zettle (PayPal) Terminal

- SECURITY: check_payment_status() no longer returns the latest unrelated purchase as 'succeeded'. It could mark THIS order paid using a previous unrelated payment; it now reports 'unknown' (the WebSocket result in initiate_cloud_payment is authoritative).
- SECURITY: cancel_cloud_payment() reports the operation as unsupported instead of falsely reporting a still-completing payment as cancelled; 'cancel_payment' removed from the advertised reader_actions accordingly.
- SECURITY: OAuth error responses no longer log the raw body (which can echo the assertion/credential); only the HTTP status is logged.
- WebSocket payment amount uses the currency's minor unit with ROUND_HALF_UP (zero-decimal currencies like JPY handled) instead of int(amount*100).

- Initial release with Zettle Reader Connect API support
- Cloud-based payment initiation via WebSocket sessions
- Reader linking and discovery
- Real-time payment progress via WebSocket events
- Payment cancellation support
- OAuth 2.0 assertion grant authentication
- Support for PayPal Reader hardware