Download the annotated evidence crop for a defect_evidence row

Renders the downloadable artifact for one defect_evidence row on demand. When the row carries a bbox, the response is the ORIGINAL photo cropped to a square window with 30% margin around the bbox and the bbox stroked in the defect's priority color — exactly what the platform's evidence viewer shows on screen. When the row has no bbox (non-CVAT-sourced evidence), the original object is streamed unmodified. Replaces the retired stored crop derivatives.