refactor(P1): complete pipeline cleanup — M1 dead code + M3 blender split

M1 dead code removal:
- admin.py: remove VALID_STL_QUALITIES + stl_quality (7 locations)
- frontend: remove stl_quality from 6 files (api/orders.ts, api/worker.ts,
  WorkerActivity.tsx, RenderInfoModal.tsx, helpTexts.ts, mocks/handlers.ts)
- blender_render.py: delete _mark_sharp_and_seams() — dead, never called (62 lines)
- step_processor.py: delete _render_via_service() + 2 elif renderer=="threejs" branches
- renderproblems_tmp/: remove 3 orphaned debug images

M3 blender_render.py decomposition (858 → 248 lines):
- _blender_gpu.py: activate_gpu(), configure_engine()
- _blender_import.py: import_glb(), apply_rotation()
- _blender_materials.py: FAILED_MATERIAL_NAME, assign_failed_material(),
  build_mat_map_lower(), apply_material_library()
- _blender_camera.py: setup_auto_camera(), setup_auto_lights()
- _blender_scene.py: ensure_collection(), apply_smooth_batch(),
  apply_sharp_edges_from_occ(), setup_shadow_catcher()
- Entry-point: sys.path.insert for submodule discovery; arg-parse + Mode A/B orchestration only

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

backend/app/services/step_processor.py

@@ -517,11 +517,6 @@ def _generate_thumbnail(
             "width": 512,
             "height": 512,
         })
-    elif renderer == "threejs":
-        # Three.js renderer removed in v2; treat as pillow fallback
-        renderer = "pillow"
-        render_log.update({"renderer": "pillow", "threejs_removed": True})
-
     logger.info(f"Thumbnail renderer={renderer}, format={fmt}")
 
     rendered_png: Path | None = None
@@ -587,41 +582,6 @@ def _finalise_image(src: Path, dst: Path, fmt: str) -> Path | None:
         return dst
 
 
-def _render_via_service(
-    url: str, step_path: Path, out_path: Path, extra: dict | None = None,
-    job_id: str | None = None,
-) -> tuple[Path | None, dict]:
-    """Call an external renderer microservice to generate a thumbnail.
-
-    Returns (path_or_None, response_data_dict).
-    job_id, when provided, is forwarded to the renderer so the render process
-    can be cancelled via the renderer's /cancel/{job_id} endpoint.
-    """
-    try:
-        import httpx
-        payload = {
-            "step_path": str(step_path),
-            "output_path": str(out_path),
-            "width": 512,
-            "height": 512,
-            **(extra or {}),
-        }
-        if job_id:
-            payload["job_id"] = job_id
-        resp = httpx.post(url, json=payload, timeout=300.0)
-        data = {}
-        try:
-            data = resp.json()
-        except Exception:
-            pass
-        if resp.status_code == 200 and out_path.exists():
-            return out_path, data
-        logger.warning(f"Renderer service {url} returned {resp.status_code}: {resp.text[:500]}")
-    except Exception as exc:
-        logger.warning(f"Renderer service {url} unreachable: {exc}")
-    return None, {}
-
-
 def _generate_thumbnail_placeholder(step_path: Path, out_path: Path, fmt: str = "png") -> Path | None:
     """Generate a simple placeholder thumbnail using Pillow."""
     try:
@@ -897,9 +857,6 @@ def render_to_file(
                 rendered_png = None
         else:
             logger.warning("Blender not available in this container — using Pillow fallback")
-    elif renderer == "threejs":
-        # Three.js renderer removed in v2 — fall through to Pillow placeholder
-        logger.warning("Three.js renderer removed; using Pillow fallback")
 
     if service_data:
         for key in ("total_duration_s", "stl_duration_s", "render_duration_s",