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/alembic/versions/039_cad_mesh_attributes.py

@@ -0,0 +1,19 @@
+"""Add mesh_attributes JSONB column to cad_files.
+
+Revision ID: 039
+Revises: 038
+"""
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects.postgresql import JSONB
+
+revision = '039'
+down_revision = '038'
+branch_labels = None
+depends_on = None
+
+def upgrade():
+    op.add_column('cad_files', sa.Column('mesh_attributes', JSONB, nullable=True))
+
+def downgrade():
+    op.drop_column('cad_files', 'mesh_attributes')