[Feature Request] Support for Terminal Transaction Webhooks

Start by reviewing the OpenAPI specification's webhook definitions and the /transactions/history endpoint referenced in the issue. Compare existing checkout webhook events with terminal transaction data, then define the event and payload needed for successful physical-terminal transactions. Done means the specification documents the new event and its transaction, receipt, and line-item fields.

Currently, the OpenAPI specification and the Webhook implementation seem to be focused on checkouts (Remote/Online Payments). For developers building e.g. a Kitchen Display Systems or real-time inventory trackers for physical points of sale (eg. using Solo), there is no push-notification for successful terminal transactions.

We are currently forced to poll the /transactions/history endpoint every few seconds to detect new sales. This is inefficient for both our infrastructure and SumUp's API performance.

I would like to request a new webhook event type, for example transaction.successful or receipt.created, that triggers as soon as a physical terminal transaction is completed.

The payload should ideally include:

• transaction_id
• amount
• currency
• status
• tip
• Link to the receipt_data or the included line items.
  • products

Real-time capability is crucial for hospitality and catering environments (e.g., "McDonald's style" order number displays). Adding this to the OpenAPI spec would allow a much broader range of automation tools to be built on top of the SumUp ecosystem.