Import card transactions

Imports a batch of up to 100 external card transactions. Each row is validated and created independently, so one bad row does not affect the others -- the response has one result per row, in the same order as the request. Larger batches are rejected as a whole. 'providerId' is the row's idempotency key: a row whose providerId was already imported comes back as SKIPPED_DUPLICATE with the existing transaction id. Always pass the issuer's own transaction id when you have one; when omitted, the key is derived from the row's card, amount, date and merchant, so re-sending the same rows is safe but two genuinely different transactions with identical content in one request are kept apart only by their order. Amounts are in minor units (cents). 'amount' and 'currency' are what settled on the card's balance account and must be in that account's currency; for a purchase made in another currency, pass the purchase-side amount as 'originalAmount' and 'originalCurrency'. Rules, each failing only that row: both or neither; the same currency must carry the same amount; both amounts are positive magnitudes and 'direction' carries the sign. When omitted they default to the settled amount. Each result echoes the row's index and providerId; a FAILED row carries the error as text and as a structured 'failure'.