Register a payment made on the terminal

Step 2 of the self-service payment terminal flow. Each element of `paymentData` creates a posted (`signed = true`) transaction against the account addressed by `invoiceId`: - `paymentType: cash` -> `paykind_id = 2` (cash), `bankCard` -> `paykind_id = 3` (credit card); - `finoperation_id` is derived from the account kind: `2` for a membership, `4` for a bill; - `firm_id` is taken from `merchantId` when present; - the transaction organization is the club addressed by `gymId`, i.e. the club the terminal stands in; - `payed_at` on a fully paid bill is set automatically. All positions of a request are written in a single database transaction: either everything is registered or nothing is. **Idempotency.** Every transaction is stamped with `notes = "TERMINAL#"`. If a payment with the same `transactionId` already exists for the client, the endpoint answers `{"result":"ok"}` without creating anything — safe to retry after a network failure. Fiscalisation is not performed on the LuckyFit side: the receipt is printed by the terminal itself.