refactor(P1): M1 dead code removal + M3 blender_render.py split

M1 — dead code removed:
- Delete blender-renderer/ and threejs-renderer/ source files
- Remove PIL/Pillow fallback block from step_processor.py
  (_generate_thumbnail_placeholder, _finalise_image JPG path)
- Remove stl_quality param from render_blender.py, render_still_task,
  render_turntable_task (was always "low"; hardcode deflection values)
- render_turntable_task now reads scene_linear/angular_deflection from
  system_settings (consistent with export_glb.py pipeline)

M3 — blender_render.py split from 263 → 68 lines:
- _blender_args.py: parse_args() — all 25 positional + named args
- _blender_scene_setup.py: setup_scene() — MODE A/B including USD import
- _blender_render_config.py: configure_and_render() — engine + output

Post-review fixes:
- _db_engine.dispose() after settings read in render_turntable_task
- _finalise_image() fmt param removed (always PNG; PIL never installed)
- _blender_import.py committed together with new submodules to satisfy
  import_usd_file dependency

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

render-worker/scripts/_blender_import.py

@@ -83,3 +83,9 @@ def apply_rotation(parts: list, rx: float, ry: float, rz: float) -> None:
     bpy.context.view_layer.objects.active = parts[0]
     bpy.ops.object.transform_apply(location=False, rotation=True, scale=False)
     print(f"[blender_render] applied rotation ({rx}°, {ry}°, {rz}°) to {len(parts)} parts")
+
+
+def import_usd_file(usd_path: str) -> list:
+    """Import USD stage into current Blender scene — delegates to import_usd module."""
+    from import_usd import import_usd_file as _impl
+    return _impl(usd_path)