Pin xgboost<3 to skip nvidia-nccl-cu12 (294MB)

backend/requirements-docker.txt

@@ -6,7 +6,7 @@ numpy>=1.24
 scikit-learn>=1.3
 requests>=2.28
 python-dotenv>=1.0
-xgboost>=2.0
+xgboost>=2.0,<3.0  # pin <3 to avoid nvidia-nccl-cu12 (294MB, useless on CPU)
 pvlib>=0.10.0
 astral>=3.2
 google-genai>=1.0