feat: video support in huddle calls #6384

Motivation
Teams that have moved their standups and working sessions into Buzz huddles (our team runs all its calls there now) still have to leave the platform whenever a call needs faces or screen-sharing — pairing, demos, design reviews, or just seeing teammates. Audio-only huddles cover a lot, but video is the remaining reason to keep a Zoom/Meet dependency next to an otherwise self-hosted, self-sovereign workspace.

Proposed solution
Add video (camera, and ideally screen-share) to huddles. Roughly in the spirit of the existing design:

• Extend the huddle transport beyond the current Opus-over-WebSocket relay (src/audio/ in buzz-relay) with a video track — either a new frame type on the same socket or a parallel /huddle/{channel_id}/video endpoint, keeping the relay forwarding opaque frames between authenticated, membership-checked peers as it does today.
• Reserve/define the corresponding Nostr event kinds for video track lifecycle, consistent with how huddle join/leave/end events already work.
• Bandwidth is the hard part: pure fan-out relay of video frames scales worse than audio, so this may justify simulcast or an optional SFU mode for larger rooms, even though audio deliberately avoids one.

Even a v1 limited to small rooms (2–5 participants, single resolution, no screen-share) would remove the external-tool dependency for most day-to-day calls.

Alternatives considered

• Keep using Zoom/Meet/Jitsi alongside Buzz — works, but splits identity, membership, and (eventually) recording/transcription across systems, and defeats the self-hosted premise.
• Embed/link an external self-hosted Jitsi or LiveKit per channel — possible today with no Buzz changes, but participants lose NIP-42-authenticated membership gating and the signed huddle event trail.
• WebRTC P2P mesh negotiated over Nostr events, bypassing the relay — avoids relay bandwidth but breaks the "all reads and writes flow through the relay" architecture and behaves badly behind NAT without TURN.

Additional context

• ARCHITECTURE.md "Huddle Audio — WebSocket Opus Relay": https://github.com/block/buzz/blob/main/ARCHITECTURE.md — notes recording and per-track publishing are planned with kinds reserved; video tracks could slot into that same track model.
• Duplicate search: none found — open "video" issues (Support streamed 2 GB mobile video uploads #4143, Video upload always fails, and the desktop swallows the reason ("Unknown error") #5193, Mobile's own video transcode produces MP4s the relay rejects (422 media contains metadata) #5415, etc.) all concern video file uploads/validation, not real-time calls.

(Filed from my account with the help of my Buzz agent, which drafted this after digging through the architecture docs.)