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

docs/uml/call-sequence.puml

@@ -40,6 +40,12 @@ note over User, Mic
   can ring even where microphone access needs a tap to be granted
 end note
 
+note over Card
+  The card opens an audio connection to the camera and holds it only while the
+  call is in progress. If the camera has no 2-way audio, or another application
+  is already using it, the call fails with an error
+end note
+
 Card --> User : You hear the caller
 note over Card
   Inbound audio unmutes if configured (<color:#43a047>**""live.auto_unmute""**</color>)
@@ -61,6 +67,7 @@ note over User, Card
   <color:#43a047>**""live.controls.call.lock""**</color> is disabled -- navigating away
 end note
 
+note over Card : The audio connection to the camera closes, releasing it for other applications
 Card --> User : Inbound audio mutes (<color:#43a047>**""live.auto_mute""**</color>)
 Card -> Mic : Microphone mutes and disconnects
 note over Mic