feat(admin): add "Regenerate All GLB + USD" button

New endpoint POST /admin/settings/regenerate-all-canonical-scenes
queues GLB + USD master export for ALL completed CAD files, replacing
existing assets. Used after pipeline changes that affect tessellation
or normals.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/routers/admin.py

@@ -607,6 +607,28 @@ async def generate_missing_usd_masters(
     return {"queued": queued, "message": f"Queued {queued} missing USD master task(s)"}
 
 
+@router.post("/settings/regenerate-all-canonical-scenes", status_code=status.HTTP_202_ACCEPTED)
+async def regenerate_all_canonical_scenes(
+    admin: User = Depends(require_global_admin),
+    db: AsyncSession = Depends(get_db),
+):
+    """Re-queue GLB + USD master export for ALL completed CAD files (overwrites existing assets)."""
+    result = await db.execute(
+        select(CadFile).where(CadFile.processing_status == ProcessingStatus.completed)
+    )
+    cad_files = result.scalars().all()
+
+    from app.tasks.step_tasks import generate_gltf_geometry_task
+    queued = 0
+    for cad_file in cad_files:
+        if not cad_file.stored_path:
+            continue
+        generate_gltf_geometry_task.delay(str(cad_file.id))
+        queued += 1
+
+    return {"queued": queued, "message": f"Queued {queued} canonical scene regeneration task(s)"}
+
+
 @router.post("/settings/recover-stuck-processing", status_code=status.HTTP_200_OK)
 async def recover_stuck_processing(
     admin: User = Depends(require_global_admin),