Record Host Onboarding Event

Record what the host application knows about one user's activation.

Args: request: The target, the host's user id, and what it is reporting. auth: The calling organization, from the X-Api-Key header. target_repo: TargetRepository dependency. chat_template_repo: ChatTemplateRepository dependency. db_session: Session used to resolve the target and its template.

Returns: The run's facts and completed steps after the event.

Raises: HTTPException: 404 when the target does not exist, is not the caller's, or runs no checklist — one status for all three on purpose, so the endpoint cannot be used to discover which targets belong to another organization. 400 when the payload names no work to do or carries no usable identity.