fix: material override pipeline — pass --material-override CLI arg to Blender scripts

The initial implementation only overrode the material_map dict in the task,
but the Blender USD primvar path bypassed it. Now:
- Added --material-override named CLI arg parsed in _blender_args.py
- Both Mode A (factory) and Mode B (template) in _blender_scene_setup.py
  override usd_material_lookup and material_map when set
- Passed through full chain: task → step_processor → render_blender → CLI → Blender
- Tested: 175-part bearing rendered with single Steel-Bare material (1/1 materials)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/step_processor.py

@@ -893,6 +893,7 @@ def render_to_file(
     tessellation_engine: str | None = None,
     focal_length_mm: float | None = None,
     sensor_width_mm: float | None = None,
+    material_override: str | None = None,
 ) -> tuple[bool, dict]:
     """Render a STEP file to a specific output path using current system settings.
 
@@ -1031,6 +1032,7 @@ def render_to_file(
                     tessellation_engine=tessellation_engine or settings["tessellation_engine"],
                     focal_length_mm=focal_length_mm,
                     sensor_width_mm=sensor_width_mm,
+                    material_override=material_override,
                 )
                 rendered_png = tmp_png if tmp_png.exists() else None
             except Exception as exc: