Updates the name and/or parent of an existing tag in the authenticated account. The tag is identified by tag_uuid in the URL path. The submitted tag_name is normalized server-side before storage with the same rules as /tags/create_tag (~ -> space, runs of , -> -, Unicode control/format characters stripped, whitespace trimmed); a tag_name empty after normalization is rejected with 422. Setting a tag as its own parent is rejected with 422. If another tag in the account already has the submitted name, 409 is returned with the conflicting tag's UUID and name in data so callers can recover without a separate lookup.