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/components/cad/InlineCadViewer.tsx

@@ -31,7 +31,7 @@ function GlbModel({ url, wireframe }: { url: string; wireframe: boolean }) {
       if (obj instanceof THREE.Mesh && obj.geometry) {
         let geo = obj.geometry.clone()
         if (!geo.index) {
-          // Non-indexed geometry (STL→GLB via trimesh): each triangle has unique vertices,
+          // Non-indexed geometry: each triangle has unique vertices,
           // so computeVertexNormals() would give per-face normals (flat shading).
           // mergeVertices() creates an indexed geometry with shared vertices first,
           // so the subsequent normal computation averages across adjacent faces → smooth.
@@ -208,7 +208,7 @@ export default function InlineCadViewer({
         className="btn-secondary text-xs"
         onClick={() => generateMut.mutate()}
         disabled={generateMut.isPending || generating}
-        title="Export STL to GLB and load 3D viewer"
+        title="Export geometry GLB from STEP via OCC and load 3D viewer"
       >
         <RefreshCw size={12} className={generating ? 'animate-spin' : ''} />
         {generating ? 'Generating…' : generateMut.isPending ? 'Queuing…' : 'Load 3D Model'}