refactor(P1): complete pipeline cleanup — M1 dead code + M3 blender split

M1 dead code removal:
- admin.py: remove VALID_STL_QUALITIES + stl_quality (7 locations)
- frontend: remove stl_quality from 6 files (api/orders.ts, api/worker.ts,
  WorkerActivity.tsx, RenderInfoModal.tsx, helpTexts.ts, mocks/handlers.ts)
- blender_render.py: delete _mark_sharp_and_seams() — dead, never called (62 lines)
- step_processor.py: delete _render_via_service() + 2 elif renderer=="threejs" branches
- renderproblems_tmp/: remove 3 orphaned debug images

M3 blender_render.py decomposition (858 → 248 lines):
- _blender_gpu.py: activate_gpu(), configure_engine()
- _blender_import.py: import_glb(), apply_rotation()
- _blender_materials.py: FAILED_MATERIAL_NAME, assign_failed_material(),
  build_mat_map_lower(), apply_material_library()
- _blender_camera.py: setup_auto_camera(), setup_auto_lights()
- _blender_scene.py: ensure_collection(), apply_smooth_batch(),
  apply_sharp_edges_from_occ(), setup_shadow_catcher()
- Entry-point: sys.path.insert for submodule discovery; arg-parse + Mode A/B orchestration only

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

frontend/src/components/renders/RenderInfoModal.tsx

@@ -141,7 +141,6 @@ export default function RenderInfoModal({
                 )}
                 {rl.format && <Row label="Format" value={rl.format.toUpperCase()} />}
                 {rl.parts_count != null && <Row label="Parts" value={rl.parts_count} />}
-                {rl.stl_quality && <Row label="STL Quality" value={rl.stl_quality} />}
                 {rl.smooth_angle != null && <Row label="Smooth Angle" value={`${rl.smooth_angle}°`} />}
               </div>
             </div>