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>

.env.example

@@ -26,3 +26,10 @@ MAX_UPLOAD_SIZE_MB=500
 # Celery worker concurrency (default: 8 parallel CAD jobs per worker container)
 # Scale horizontally with: docker compose up --scale worker=N
 CELERY_WORKER_CONCURRENCY=8
+
+# Blender (render-worker)
+# Blender >= 5.0.1 must be installed on the host at /opt/blender
+# The render-worker container mounts it read-only via volumes: - /opt/blender:/opt/blender:ro
+BLENDER_VERSION=5.0.1
+# Set to host path if Blender is not at /opt/blender:
+# BLENDER_BIN=/usr/local/blender/blender