Create many exemptions

Create between 1 and 100 exemptions in the resolved organization in one request. The batch is all-or-nothing: every entry is validated first, and if any one is invalid the whole request returns 400 identifying the offending entry's index and nothing is created. Each entry follows the same rules as POST /exemptions: customerId is required and must belong to the organization, as must transactionId when sent, and source is always recorded as API. A partial exemption is not accepted here; create it with POST /exemptions. An empty list, or more than 100 entries, returns 400.