Detect and stream if a chair is occupied
For a soft seat you could sense occupancy with an LED and photo-sensor at opposite ends of a short somewhat flexible tube that gets flexed when the cushion compresses, interrupting the light. That or a microswitch would have to be the cheapest means of sensing. 

Getting from there to the server depends a little on how much work you're willing to do, from rolling your own micro-controller+WiFi PCB to purchasing off the shelf parts, which may in turn, depend on whether you need to monitor one chair or a whole auditorium.