Record that the user applied or dismissed a proposal

The frontend applies proposals through its existing flows (widgets through the dashboard page state, a report through POST /reports/generations) and then reports the outcome here, so the proposal part's status is persisted. pending → applied | dismissed; repeating the current status is accepted; changing a settled proposal is a 409. result is an optional JSON object stored with the proposal (for example the created report's id). Requires org.view_dashboard; 404 for another user's proposal.