Smart Human Detection System Development

I need a reliable human-detection system built from the ground up. The core requirement is straightforward: accurately recognise when a person enters the monitored zone and trigger a software event that I can later tie into alerts, recording, or access-control logic... (Budget: $250 - $750 USD, Jobs: AI Integration, AI Model Development, Computer Vision, Deep Learning, Machine Learning (ML), Neural Networks, Object Detection, OpenCV)