Get the group a post belongs to, creating it if this is the first thing made from that post.

This is the first call in a repurposing pipeline: seed the group from the post, then pass the id as bucketId on every POST /v1/content derived from it. The writer then sees the post and its versions as one row under Content instead of unrelated drafts.

Safe to call repeatedly — a post that already has a group gets the same id back, and nothing is written.