Request approvals

Start an existing pending approval flow by emailing its initial approver(s) and recording who requested the approvals. This is the V3 equivalent of the in-app "Request Approval" button: until it is called, an approval flow's approver groups exist but no one is asked to approve. The flow must be startable — every group already populated, no resolved approvers yet, and (for offer_candidate flows) an offer_id set — otherwise the call returns 422. Pass requested_by_user_id to record someone other than the calling user as the requester; the same user is shown as the sender on the approval emails. Returns 204 with no body.