refactor: replace STL intermediary with OCC-native STEP→GLB pipeline

- export_step_to_gltf.py: STEP→GLB via RWGltf_CafWriter + BRepBuilderAPI_Transform
  (mm→m pre-scaling, XCAFDoc_ShapeTool.GetComponents_s static method)
- Blender scripts (blender_render.py, still_render.py, turntable_render.py,
  export_gltf.py, export_blend.py): import GLB instead of STL, remove _scale_mm_to_m
- step_tasks.py: add generate_gltf_production_task, remove generate_stl_cache,
  replace _bbox_from_stl with _bbox_from_glb (trimesh), auto-queue geometry GLB
  after thumbnail render
- render_blender.py: replace _stl_from_cache_or_convert with _glb_from_step,
  remove convert_step_to_stl and export_per_part_stls
- domains/rendering/tasks.py: update render_turntable_task, export_gltf/blend tasks
  to use GLB instead of STL
- cad.py: remove STL download/generate endpoints, add generate-gltf-production
- admin.py: generate-missing-stls → generate-missing-geometry-glbs
- Frontend: replace STL cache UI with GLB generate buttons, remove stl_cached field

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

frontend/src/pages/CadPreview.tsx

@@ -96,8 +96,8 @@ export default function CadPreviewPage() {
           <Box size={48} className="text-gray-600" />
           <p className="text-white text-lg font-semibold">No 3D model available yet</p>
           <p className="text-gray-400 text-sm max-w-sm">
-            Generate a GLB file from the STEP cache to enable the 3D viewer.
-            The STL cache must exist (process the STEP file first).
+            Generate a geometry GLB from the STEP file to enable the 3D viewer.
+            Process the STEP file first to make it available.
           </p>
           {generating ? (
             <div className="flex items-center gap-2 text-gray-300 text-sm">
@@ -116,7 +116,7 @@ export default function CadPreviewPage() {
           )}
           {generateMutation.isError && (
             <p className="text-red-400 text-sm">
-              Failed to start generation. Check that the STL cache exists.
+              Failed to start generation. Make sure the STEP file has been processed.
             </p>
           )}
         </div>