fix: animation output types now render turntable mp4 via Celery

- render_order_line_task: detect is_animation flag, branch to turntable
  pipeline instead of still-image pipeline
- render_blender: add render_turntable_to_file() — STL conversion,
  blender turntable_render.py for frames, ffmpeg compose to mp4
- render-worker/Dockerfile: add ffmpeg package
- Fix: Turntable Video_White was producing a .jpg still instead of .mp4

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

render-worker/Dockerfile

@@ -44,6 +44,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     libegl1 \
     libegl-mesa0 \
     libgbm1 \
+    ffmpeg \
     && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app