Add read endpoints for binary assets so the iPad can preview authored parts and signs it didn't just upload.\n\nEndpoints:\n- GET /parts/{id}/glb — download a part's GLB\n- GET /signs/{id}/png — download a sign's full PNG\n- GET /signs/{id}/thumb — thumbnail (if not already served)\n\nBlocks iPad part authoring preview. Discovered from #298.
Add read endpoints for binary assets so the iPad can preview authored parts and signs it didn't just upload.\n\nEndpoints:\n-
GET /parts/{id}/glb— download a part's GLB\n-GET /signs/{id}/png— download a sign's full PNG\n-GET /signs/{id}/thumb— thumbnail (if not already served)\n\nBlocks iPad part authoring preview. Discovered from #298.