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/components/live/carousel.ts

@@ -265,14 +265,11 @@ export class AdvancedCameraCardLiveCarousel extends LitElement {
     const mediaEpoch = view?.context?.mediaEpoch?.[cameraID] ?? 0;
 
     const isSelectedSlide = !!view?.camera && cameraID === view.camera;
-    const microphoneStream = this._getRelevantMicrophoneStream(cameraID, view);
-
     return html`
       <div class="embla__slide">
         ${keyed(
           mediaEpoch,
           html`<advanced-camera-card-live-provider
-            .microphoneStream=${microphoneStream}
             .camera=${resolvedCamera}
             .targetID=${cameraID}
             .cameraTitle=${cameraMetadata?.title}
@@ -304,25 +301,6 @@ export class AdvancedCameraCardLiveCarousel extends LitElement {
     return view?.context?.live?.overrides?.get(cameraID) ?? cameraID;
   }
 
-  // Return a microphone stream only for the camera the call runs on, only
-  // while the call has been answered, and only while that camera's engaged
-  // stream is still the call's audio source. The `answered` gate is a
-  // privacy guarantee: an inbound call that's still ringing must not
-  // transmit audio even if the mic happens to be un-muted (e.g. left open
-  // by `auto_unmute: ['selected']` or a prior call). The substream gate
-  // stops transmission if the substream has since changed.
-  private _getRelevantMicrophoneStream(
-    cameraID: string,
-    view?: View | null,
-  ): MediaStream | null {
-    const isRelevant =
-      !!this.call?.answered &&
-      this.call.cameraID === cameraID &&
-      this._getSubstreamCameraID(cameraID, view) ===
-        (this.call.callCameraID ?? cameraID);
-    return isRelevant ? this.microphoneState?.stream ?? null : null;
-  }
-
   private _toggleMute(): void {
     const controller = this._mediaLoadedInfoSinkController.get()?.mediaPlayerController;
     // Fire-and-forget; the `volumechange` event drives the re-render.