Create or update contacts in bulk

Up to 500 contacts per call, each created or updated by phone or BSUID. The answer is 200 even when rows fail: each row's outcome comes in results, in the order sent.

On an update, only filled values overwrite and tags are added. With options.optIn, new contacts are born with consent and existing ones only get it if they were still unknown — an opt-out is never undone. A batch fires no webhooks.

An API key limited to some numbers gets 403: this operation covers the whole base.