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>

backend/app/api/routers/admin.py

@@ -359,24 +359,20 @@ async def generate_missing_stls(
 async def renderer_status(
     admin: User = Depends(require_admin),
 ):
-    """Check health of external renderer services."""
-    import httpx
-    services = {
-        "pillow": {"url": None, "available": True, "note": "Built-in (always available)"},
-        "blender": {"url": "http://blender-renderer:8100/health", "available": False, "note": ""},
+    """Check health of renderer services."""
+    from app.services.render_blender import find_blender, is_blender_available
+    blender_available = is_blender_available()
+    blender_bin = find_blender()
+    return {
+        "pillow": {"available": True, "note": "Built-in (always available)"},
+        "blender": {
+            "available": blender_available,
+            "note": (
+                f"render-worker subprocess ({blender_bin})"
+                if blender_available
+                else "Blender not found — check render-worker container and BLENDER_BIN"
+            ),
+        },
     }
-    async with httpx.AsyncClient(timeout=3.0) as client:
-        for name, info in services.items():
-            if info["url"] is None:
-                continue
-            try:
-                resp = await client.get(info["url"])
-                if resp.status_code == 200:
-                    data = resp.json()
-                    services[name]["available"] = True
-                    services[name]["note"] = data.get("renderer", name)
-            except Exception as e:
-                services[name]["note"] = str(e)[:100]
-    return services