Add Contacts Bulk

Add multiple contacts in a single request.

This is much more efficient than calling the single contact endpoint multiple times, as it reduces the number of database roundtrips and auth checks.

Duplicates (matching an existing contact — or an earlier row of the same batch — by person_id, email, or slack_user_id) are skipped, not created: on an active campaign every created row is invited immediately, and the client's own duplicate check is a stale snapshot (a teammate may have added the same person since, or a retried request would double-send).