Create a prefilled DRAFT offer for a candidate on a position. positionId and profileId are required; applicationId is optional -- when omitted, the candidate's latest application to the position is linked. The candidate must already have an application to the position; if they have none the request is rejected (advance the candidate into the position's pipeline first via the application-stages API). Field values are set in a separate PATCH /offers/ call (two-step create, mirroring the UI create -> intake-form flow). Returns the full created offer. If an offer already exists for the candidate+position, that existing offer is returned.