feat(D): OCC mesh attribute extraction + Blender smooth shading integration

Migration 039: cad_files.mesh_attributes JSONB column.
domains/products/tasks.py: extract_mesh_attributes Celery task using pythonOCC.
still_render.py + turntable_render.py: _apply_mesh_attributes() sets auto-smooth
based on curved_ratio and topology threshold from OCC analysis.
render_blender.py: passes --mesh-attributes JSON arg to Blender subprocess.
render_still_task: loads mesh_attributes from DB and passes to renderer.

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

backend/app/tasks/celery_app.py

@@ -9,6 +9,7 @@ celery_app = Celery(
         "app.tasks.step_tasks",
         "app.tasks.ai_tasks",
         "app.domains.rendering.tasks",
+        "app.domains.products.tasks",
     ],
 )