Pin onnxruntime to 1.20.1 to address SIGILL on certain arm64 hosts

backend/requirements.txt

@@ -83,6 +83,8 @@ opencv-python-headless==4.11.0.86
 rapidocr-onnxruntime==1.3.24
 rank-bm25==0.2.2
 
+onnxruntime==1.20.1
+
 faster-whisper==1.1.1
 
 PyJWT[crypto]==2.10.1