Hard-delete an estimate. Allowed only when the estimate is provably inert — a EstimateStatus.Draft that has never been converted and has no line that has performed/dispensed anything. Anything with committed side effects (dispensed stock, DEA entries, a cut Rx) is refused with 409: the raw cascade delete reverses none of that, so those lines must be voided first (which restores stock, files the DEA correction, cancels the Rx). Mirrors the per-line delete guard at Task<ActionResult> EstimatesController.DeleteItemAsync(string id, string lineId).