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/camera-manager/frigate/camera.ts

@@ -3,6 +3,10 @@ import { format } from 'date-fns';
 import type { ActionsExecutor } from '../../card-controller/actions/types';
 import type { PTZAction, PTZActionPhase } from '../../config/schema/actions/custom/ptz';
 import type { CameraConfig } from '../../config/schema/cameras';
+import {
+  getGo2RTCMetadataEndpoint,
+  getGo2RTCStreamEndpoint,
+} from '../../go2rtc/endpoint';
 import type { Entity, EntityRegistryManager } from '../../ha/registry/entity/types';
 import type { HomeAssistant } from '../../ha/types';
 import {
@@ -16,10 +20,6 @@ import { Camera, type CameraInitializationOptions } from '../camera';
 import { CameraNoEntityError } from '../error';
 import type { CameraEndpoints, CameraEndpointsContext } from '../types';
 import { getCameraEntityFromConfig } from '../utils/camera-entity-from-config';
-import {
-  getGo2RTCMetadataEndpoint,
-  getGo2RTCStreamEndpoint,
-} from '../utils/go2rtc/endpoint';
 import { getPTZCapabilitiesFromCameraConfig, mergePTZCapabilities } from '../utils/ptz';
 import { getPTZInfo } from './requests';
 import {