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_scene_setup.py

@@ -0,0 +1,129 @@
+"""MODE A / MODE B scene setup for blender_render.py.
+
+MODE A — factory settings (no template): auto-camera + auto-lights
+MODE B — template file: load .blend, import into named collection
+"""
+import time as _time
+from typing import Callable
+
+import bpy  # type: ignore[import]
+
+from _blender_camera    import setup_auto_camera, setup_auto_lights
+from _blender_import    import import_glb, apply_rotation, import_usd_file
+from _blender_materials import (
+    assign_failed_material,
+    build_mat_map_lower,
+    apply_material_library,
+)
+from _blender_scene     import (
+    ensure_collection,
+    apply_smooth_batch,
+    apply_sharp_edges_from_occ,
+    setup_shadow_catcher,
+)
+
+
+def setup_scene(args, lap_fn: Callable[[str], None]) -> None:
+    """Set up the Blender scene according to args (MODE A or B).
+
+    Handles import, rotation, smooth shading, material assignment, shadow
+    catcher, and auto-camera/lights. lap_fn is called with a label string
+    at each timing checkpoint.
+    """
+    if args.use_template:
+        _setup_mode_b(args, lap_fn)
+    else:
+        _setup_mode_a(args)
+
+
+def _setup_mode_b(args, lap_fn: Callable[[str], None]) -> None:
+    """MODE B: Template-based render — load .blend, import into collection."""
+    print(f"[blender_render] Opening template: {args.template_path}")
+    bpy.ops.wm.open_mainfile(filepath=args.template_path)
+    lap_fn("template_load")
+
+    target_col = ensure_collection(args.target_collection)
+    if args.usd_path:
+        parts = import_usd_file(args.usd_path)
+    else:
+        parts = import_glb(args.glb_path)
+    lap_fn("glb_import")
+    apply_rotation(parts, args.rotation_x, args.rotation_y, args.rotation_z)
+    lap_fn("rotation")
+
+    for part in parts:
+        for col in list(part.users_collection):
+            col.objects.unlink(part)
+        target_col.objects.link(part)
+
+    apply_smooth_batch(parts, args.smooth_angle)
+    if not args.usd_path:
+        _occ_pairs = args.mesh_attributes.get("sharp_edge_pairs") or []
+        if _occ_pairs:
+            apply_sharp_edges_from_occ(parts, _occ_pairs)
+    lap_fn("smooth_shading")
+
+    if args.material_library_path and args.material_map:
+        apply_material_library(
+            parts, args.material_library_path,
+            build_mat_map_lower(args.material_map), args.part_names_ordered,
+        )
+    else:
+        for part in parts:
+            assign_failed_material(part)
+    lap_fn("material_assign")
+
+    if args.shadow_catcher:
+        setup_shadow_catcher(parts)
+
+    needs_auto_camera = (
+        (args.lighting_only and not args.shadow_catcher)
+        or not bpy.context.scene.camera
+    )
+    if args.lighting_only and not args.shadow_catcher:
+        print("[blender_render] lighting_only mode: using template World/HDRI, forcing auto-camera")
+    elif needs_auto_camera:
+        print("[blender_render] WARNING: template has no camera — will create auto-camera")
+
+    if not needs_auto_camera and bpy.context.scene.camera:
+        bpy.context.scene.camera.data.clip_start = 0.001
+
+    print(f"[blender_render] template mode: {len(parts)} parts imported into collection '{args.target_collection}'")
+
+    if needs_auto_camera:
+        setup_auto_camera(parts, args.width, args.height)
+
+
+def _setup_mode_a(args) -> None:
+    """MODE A: Factory settings — auto-camera + auto-lights."""
+    bpy.ops.wm.read_factory_settings(use_empty=True)
+    if args.usd_path:
+        parts = import_usd_file(args.usd_path)
+    else:
+        parts = import_glb(args.glb_path)
+    apply_rotation(parts, args.rotation_x, args.rotation_y, args.rotation_z)
+
+    _t = _time.time()
+    apply_smooth_batch(parts, args.smooth_angle)
+    if not args.usd_path:
+        _occ_pairs = args.mesh_attributes.get("sharp_edge_pairs") or []
+        if _occ_pairs:
+            apply_sharp_edges_from_occ(parts, _occ_pairs)
+    for part in parts:
+        assign_failed_material(part)
+    print(f"[blender_render] smooth+fallback-material: {len(parts)} parts ({_time.time()-_t:.2f}s)", flush=True)
+
+    if args.material_library_path and args.material_map:
+        apply_material_library(
+            parts, args.material_library_path,
+            build_mat_map_lower(args.material_map), args.part_names_ordered,
+        )
+
+    bbox_center, bsphere_radius = setup_auto_camera(parts, args.width, args.height)
+    setup_auto_lights(bbox_center, bsphere_radius)
+    world = bpy.data.worlds.new("World")
+    bpy.context.scene.world = world
+    world.use_nodes = True
+    bg = world.node_tree.nodes["Background"]
+    bg.inputs["Color"].default_value    = (0.96, 0.96, 0.97, 1.0)
+    bg.inputs["Strength"].default_value = 0.15