feat(N): workflow pipeline, 3D viewer, worker management, QC tests

- workflow_builder.py: fix broken stubs, add render_order_line_still_task
  (resolves step_path from DB instead of passing order_line_id as step_path)
- domains/rendering/tasks.py: add render_order_line_still_task,
  export_gltf_for_order_line_task, export_blend_for_order_line_task,
  generate_gltf_geometry_task (trimesh STL→GLB, no Blender needed)
- tasks/step_tasks.py: add generate_gltf_geometry_task for CadFile GLB export
- cad router: POST /{id}/generate-gltf-geometry endpoint (admin/PM)
- worker router: GET /celery-workers + POST /scale (docker compose subprocess)
- Dockerfile: pip install -e "[dev]" to enable pytest
- docker-compose.yml: docker socket + compose file mount on backend
- ThreeDViewer.tsx: mode toggle (geometry/production), wireframe, env presets,
  download buttons (GLB + .blend)
- CadPreview.tsx: load gltf_geometry/gltf_production/blend_production assets
  from MediaAsset table and pass URLs to ThreeDViewer
- ProductDetail.tsx: "View 3D" button → /cad/:id, "Generate GLB" button
- media router/service: cad_file_id filter on GET /api/media
- WorkerManagement.tsx: new page with worker status, queue depth, scale controls
- App.tsx + Layout.tsx: /workers route + sidebar link (admin/PM)
- tests: test_rendering_service.py, test_orders_service.py (backend)
- tests: WorkerActivity.test.tsx, WorkerManagement.test.tsx (frontend)

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

backend/Dockerfile

@@ -1,3 +1,6 @@
+# Stage 0: grab docker CLI + compose plugin
+FROM docker:cli AS docker-cli
+
 FROM python:3.11-slim
 
 WORKDIR /app
@@ -13,9 +16,13 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     libffi-dev \
     && rm -rf /var/lib/apt/lists/*
 
-# Install Python dependencies
+# Copy docker CLI for worker scaling
+COPY --from=docker-cli /usr/local/bin/docker /usr/local/bin/docker
+COPY --from=docker-cli /usr/local/lib/docker/cli-plugins /usr/local/lib/docker/cli-plugins
+
+# Install Python dependencies (including dev extras for pytest)
 COPY pyproject.toml .
-RUN pip install --no-cache-dir -e .
+RUN pip install --no-cache-dir -e ".[dev]"
 
 # Copy app code
 COPY . .