Create question forecast

Start a forecast of the question that takes `additional_context` into account. Give facts with their source and date where you can, and label assumptions as hypothetical: the forecast weighs stated facts as evidence, treats labelled assumptions separately, and keeps the question and its resolution criteria unchanged. The forecast builds on the question's latest forecast, is marked `user_context` and is visible to you and your organisation. `GET /forecasts/{id}` returns it once it completes.

curl -X POST "https://example.com/questions/0/forecasts" \  -H "Content-Type: application/json" \  -d '{    "additional_context": "Reuters reported on 12 October that the board approved the merger. Assume regulators clear it by the end of the year."  }'

{  "id": 0}