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/configuration/cameras/live-provider.md

@@ -42,7 +42,12 @@ cameras:
 > microphone button is intermittently missing on load, try increasing
 > `metadata_fetch_timeout_seconds` or use
 > [`capabilities.force`](./README.md?id=capabilities) to skip metadata
-> detection entirely.
+> detection entirely. If it is _never_ present, check the `go2rtc` stream
+> itself: Frigate [recommends](https://docs.frigate.video/configuration/live/)
+> adding `#backchannel=0` to the stream it restreams, which stops that stream
+> offering 2-way audio at all. Point `stream` at a talk-capable stream instead.
+> Note that _any_ `#` option on an RTSP source has this effect unless it also
+> includes `backchannel=1`.
 
 ## `go2rtc (experimental)`