Add up to 500 leads to a campaign

One campaign, up to 500 items, one result per item in input order. Each item is judged exactly as POST /api/v1/leads would judge it on its own, plus duplicate_in_request for a person who appears earlier in the same request. The request is 200 whenever it is valid, even if every item was refused. A retry is safe: an item that already landed comes back duplicate. The approval check runs once for the whole request and only if at least one item is about to be added. Limits: 100 requests/minute per key (shared with every v1 route) plus 2,000 items/minute per key; both are per server instance.