Link a pre-existing open estimate to an appointment. Honors the unique index on `treatment_plans.appointment_id` (one estimate per appointment): * No estimate on the appointment yet → set AppointmentId on the chosen one. * The appointment's auto-created estimate is empty → drop it and re-point the chosen estimate (preserves the chosen estimate's identity + consent history). * The appointment's estimate already has lines → merge the chosen estimate's lines into it and remove the chosen estimate. Returns the resulting estimate the Services tab should render.