Feature request: AV1 support for MSE/fMP4 streaming

Hi, With the recent introduction of AV1 native encoding in IP cameras — most notably Axis Communications shipping AV1 as the default codec on all new ARTPEC-9 based cameras starting in 2025 — AV1 RTSP streams from cameras are becoming increasingly common in the wild.

go2rtc already supports AV1 for WebRTC, which is great. However MSE (the low-latency browser streaming path most commonly used with Frigate) currently only handles H264 and H265. When an AV1 camera stream arrives, go2rtc has no path to serve it via MSE without transcoding to H264/H265 first.

AV1 in fMP4 containers is well-standardized (ISO 14496-15, av01 box), and all major browsers — Chrome, Firefox, Edge — support AV1 MSE natively. The infrastructure is there on both the server and client side.

Would it be feasible to add AV1 to the MSE/fMP4 muxer path, similar to how H265 MSE support was added? This would allow AV1 camera streams to be served directly to browsers without any intermediate transcoding step — which is aligned with go2rtc's philosophy of codec passthrough over transcoding.

Thanks for the great project.