refactor(A2): replace blender-renderer HTTP service with render-worker Celery container

- Create render-worker/ with Dockerfile (Ubuntu + cadquery + Blender via host mount)
- Add render-worker/check_version.py: verifies Blender >= 5.0.1 at startup, Exit 1 on failure
- Add render-worker/scripts/: blender_render.py, still_render.py, turntable_render.py
- Create backend/app/services/render_blender.py: direct subprocess rendering
  - convert_step_to_stl() and export_per_part_stls() using cadquery
  - render_still(): STEP → STL → PNG via Blender subprocess
  - is_blender_available(): detects BLENDER_BIN env for render-worker context
- Create backend/app/domains/rendering/tasks.py: render_still_task + render_turntable_task
- Update step_processor.py: use subprocess path when BLENDER_BIN env is set (render-worker)
- Update step_tasks.py: generate_stl_cache uses direct cadquery instead of HTTP
- Remove blender-renderer and threejs-renderer from docker-compose.yml
- Replace worker-thumbnail with render-worker (Ubuntu + cadquery + Blender mount)
- Remove Docker SDK from backend Dockerfile (was only for flamenco scaling)
- Update .env.example: BLENDER_VERSION=5.0.1 documented
- Update celery_app.py: include domains.rendering.tasks in autodiscover

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

render-worker/Dockerfile

@@ -0,0 +1,61 @@
+FROM ubuntu:22.04
+
+ARG BLENDER_VERSION=5.0.1
+ENV DEBIAN_FRONTEND=noninteractive
+ENV PYTHONUNBUFFERED=1
+ENV BLENDER_VERSION=${BLENDER_VERSION}
+# Blender 5.x is mounted from the host at /opt/blender (see docker-compose.yml)
+ENV BLENDER_BIN=/opt/blender/blender
+# OSMesa for headless cadquery/VTK (no display needed)
+ENV PYOPENGL_PLATFORM=osmesa
+ENV VTK_DEFAULT_EGL=0
+
+# Runtime libraries for cadquery/OCC + Blender 5.x headless
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    python3-pip \
+    python3-dev \
+    libpq-dev \
+    gcc \
+    libxrender1 \
+    libxi6 \
+    libxkbcommon-x11-0 \
+    libsm6 \
+    libglib2.0-0 \
+    libgl1-mesa-glx \
+    libosmesa6 \
+    libgomp1 \
+    libxfixes3 \
+    libxrandr2 \
+    libxcursor1 \
+    libxinerama1 \
+    libwayland-client0 \
+    libwayland-cursor0 \
+    libwayland-egl1 \
+    libvulkan1 \
+    mesa-vulkan-drivers \
+    libegl1 \
+    libegl-mesa0 \
+    libgbm1 \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+
+# Install backend Python dependencies (includes celery, sqlalchemy, fastapi, etc.)
+COPY pyproject.toml .
+RUN pip3 install --no-cache-dir -e .
+
+# Install cadquery (heavy — installed after backend deps for better layer caching)
+RUN pip3 install --no-cache-dir "cadquery>=2.4.0"
+
+# Copy render scripts
+COPY scripts/ /render-scripts/
+
+# Version check script — fails fast if Blender < 5.0.1
+COPY check_version.py /check_version.py
+
+# Copy app code (overridden by volume mount in docker-compose)
+COPY . .
+
+# Verify Blender version at build time if binary is available
+# (skipped during build since /opt/blender is a host mount)
+CMD ["bash", "-c", "python3 /check_version.py && celery -A app.tasks.celery_app worker --loglevel=info -Q thumbnail_rendering --concurrency=1"]