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/render_blender.py

@@ -94,6 +94,7 @@ def render_still(
     tessellation_engine: str = "occ",
     focal_length_mm: float | None = None,
     sensor_width_mm: float | None = None,
+    material_override: str | None = None,
 ) -> dict:
     """Convert STEP → GLB (OCC or GMSH) → PNG (Blender subprocess).
 
@@ -185,6 +186,8 @@ def render_still(
             cmd += ["--focal-length", str(focal_length_mm)]
         if sensor_width_mm is not None:
             cmd += ["--sensor-width", str(sensor_width_mm)]
+        if material_override:
+            cmd += ["--material-override", material_override]
         return cmd
 
     def _run(eng: str) -> tuple[int, list[str], list[str]]:
@@ -322,6 +325,7 @@ def render_turntable_to_file(
     tessellation_engine: str = "occ",
     focal_length_mm: float | None = None,
     sensor_width_mm: float | None = None,
+    material_override: str | None = None,
 ) -> dict:
     """Render a turntable animation: STEP → STL → N frames (Blender) → mp4 (ffmpeg).
 
@@ -408,6 +412,8 @@ def render_turntable_to_file(
         cmd += ["--focal-length", str(focal_length_mm)]
     if sensor_width_mm is not None:
         cmd += ["--sensor-width", str(sensor_width_mm)]
+    if material_override:
+        cmd += ["--material-override", material_override]
 
     log_lines: list[str] = []