fix: Hold the 2-way audio backchannel open for the shortest possible time (#2697)

The card claimed a camera's ONVIF audio backchannel in two places that
had
nothing to do with a call: the capability probe at camera init
(`&microphone`),
and a pre-armed `sendonly` audio transceiver on every live WebRTC offer.
Merely
looking at a dashboard occupied the camera's speaker line. Outbound
audio now
travels on its own audio-only WebRTC connection, opened when a call is
answered
and closed when it ends.

- The backchannel is claimed only for the duration of a call. Idle
viewing
  claims nothing.
- Two-way audio now works in `mse`, `mp4` and `mjpeg` modes (note: the
outbound
  audio still traverses WebRTC).
- No renegotiation and no video blink at call start or end.
- A call that cannot carry audio now reports it and ends, instead of
showing a
  live microphone that goes nowhere.
- `live.microphone.always_connected` is now purely about the browser
microphone
  permission prompt.
- Call setup measured at 66ms (LAN) and ~260ms (cellular) for ICE and
DTLS, plus
  ~300ms for `go2rtc` to open an RTSP backchannel.

Verified against a live Frigate + `go2rtc` instance, and by unit tests
at 100%
coverage.

 - Closes #2691
 - Closes #2039
 - Closes #2178

Ref #2299 -- the probe no longer opens a backchannel, but it still runs
per
camera on every load and reconnect, and still dials the camera on the
direct-`go2rtc` path. Caching remains to be done.

Ref AlexxIT/go2rtc#1860 -- once a call has opened a backchannel,
`go2rtc` keeps
that media set up on the camera's RTSP session for the life of the
producer.

Diagnoses #2678

src/card-controller/microphone-manager.ts

@@ -101,8 +101,10 @@ export class MicrophoneManager {
 
     // A connect over an existing stream must not leak the tracks of the
     // stream it replaces.
+    this._removeEndedListeners(this._stream);
     this._stopTracks(this._stream);
     this._stream = stream;
+    this._addEndedListeners(stream);
     this._forbidden = false;
     this._reconcile();
     this._setState();
@@ -171,8 +173,31 @@ export class MicrophoneManager {
     stream?.getTracks().forEach((track) => track.stop());
   }
 
+  // A device that disappears -- unplugged, or its permission revoked -- ends
+  // its tracks. Nothing can revive them, so the stream is dropped and the new
+  // state published, rather than leaving the card reporting a connected
+  // microphone that captures nothing. `stop()` does not fire this event, so
+  // releasing the stream cannot re-enter.
+  private _handleTrackEnded = (): void => {
+    this._releaseStream();
+    this._setState();
+  };
+
+  private _addEndedListeners(stream: MediaStream): void {
+    stream
+      .getTracks()
+      .forEach((track) => track.addEventListener('ended', this._handleTrackEnded));
+  }
+
+  private _removeEndedListeners(stream: MediaStream | null): void {
+    stream
+      ?.getTracks()
+      .forEach((track) => track.removeEventListener('ended', this._handleTrackEnded));
+  }
+
   private _releaseStream(): void {
     this._connectGeneration.invalidate();
+    this._removeEndedListeners(this._stream);
     this._stopTracks(this._stream);
     this._stream = null;
   }