fix(cv2): use opencv-python-headless to remove libGL runtime dep

requirements.txt

@@ -4,7 +4,8 @@ python-multipart==0.0.9
 # NOTE: FastAPI 0.112.1 excludes pydantic 2.0.0/2.0.1/2.1.0; use a compatible 2.x
 pydantic>=2.1.1,<3
 ultralytics>=8.3.50
-opencv-python>=4.9.0.80
+# Use headless build to avoid libGL dependency in container runtime
+opencv-python-headless>=4.9.0.80
 numpy>=1.26.0
 pillow>=10.3.0
 torch>=2.2.0