Apply a tag or list change to every contact matching a filter

Queues one action against every contact the filter matches, applied in chunks in the background. Returns immediately with an id to poll.

Run with dry_run: true first. A dry run reports how many contacts match and changes nothing — it is the only way to catch a filter that matches more than intended, and a bulk write is not undoable. An empty filter (which would match the whole account) is rejected unless it is a dry run.

Per-campaign conditions (opened_campaign_id, not_opened_campaign_id, clicked_campaign_id) are available here but not on GET /contacts, because they are resolved in the background rather than inside a request.

Bulk delete and bulk unsubscribe are deliberately not offered.