Create contacts in bulk

Creates up to 1000 contacts in a single request. Each item is processed independently within its own database SAVEPOINT, so a failure on one row (e.g. missing email) does not stop the rest of the batch. The response returns a per-item status with either the created contact or an error message.

Returns 201 if at least one item was created. Returns 400 if the envelope is invalid (missing contacts array, empty array, more than 1000 items) or if every item failed.