DM4 - every question clearly belongs to a context: question.category -> question_categories.id; question_categories.contextid -> context.id.
So, there is an obvious value to add to outcome_areas.contextid for questions. That is useful metadata. In is not necessary for anything else in your spec, but I still think it should be added. (It may be useful for future reporting or maintenance things you you have not thought about yet, and won't be implemented in the first version.)