Video conferencing SFU

Design the media-routing backend for a group video-conferencing product (think 50-person calls, 5,000 concurrent meetings). Clients use WebRTC. You need <200 ms glass-to-glass latency, graceful degradation on bad networks, and the ability to scale a single large meeting beyond one server. Decide the topology (mesh vs. MCU vs. SFU), how you handle heterogeneous client bandwidth, and where the hard limits are.