feat(sfu): AudioReceiveSlotBound event (3RTT T33)

[tbarbugli]

Part of the 3RTT project (task T33, audio receive slots). Targets the project integration branch factory/3rtt, not main.

What: a new SFU event, SfuEvent.audio_receive_slot_bound = 29, with the message:

message AudioReceiveSlotBound {
  string mid = 1;
  string user_id = 2;
  string session_id = 3;
  models.TrackType track_type = 4;
}

Why: FastJoinRequest.audio_receive_slots (T20) asks the SFU for send-only audio m-lines in the FastJoin subscriber offer, before anyone publishes into them. When a participant starts publishing audio later, the SFU binds it to a free slot with ReplaceTrack. The slot keeps the SSRC and msid its offer declared, so there is no renegotiation and the client can't tell from the media whose audio it is. This event tells it: the slot's mid, and the publisher's user, session and track type.

Additive only: one new oneof member and one new message. Nothing is renumbered and no existing field changes. Clients that don't ask for slots never get the event.

Generated with the pinned toolchain (generate.sh), protolint clean, go build ./... passes. Tag: v1.50.0-3rtt.2 (on this branch's head, 99cb2f3). GitHub Actions is blocked by org billing, so CI does not run on this PR.

Used by video-sfu and getstream-go-webrtc, branch 3rtt/t33-audio-slots in each. Their PRs have the measurements.