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/services/render_blender.py

@@ -180,6 +180,7 @@ def render_still(
     denoising_prefilter: str = "",
     denoising_quality: str = "",
     denoising_use_gpu: str = "",
+    mesh_attributes: dict | None = None,
 ) -> dict:
     """Convert STEP → STL (cadquery) → PNG (Blender subprocess).
 
@@ -238,7 +239,7 @@ def render_still(
         env["EGL_PLATFORM"] = "surfaceless"
 
     def _build_cmd(eng: str) -> list:
-        return [
+        cmd = [
             blender_bin,
             "--background",
             "--python", str(script_path),
@@ -261,6 +262,9 @@ def render_still(
             denoising_input_passes or "", denoising_prefilter or "",
             denoising_quality or "", denoising_use_gpu or "",
         ]
+        if mesh_attributes:
+            cmd += ["--mesh-attributes", json.dumps(mesh_attributes)]
+        return cmd
 
     def _run(eng: str) -> subprocess.CompletedProcess:
         proc = subprocess.Popen(