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

tests/components-lib/media-dimensions-container-controller.test.ts

@@ -33,6 +33,14 @@ describe('MediaDimensionsContainerController', () => {
   });
   beforeEach(() => {
     vi.clearAllMocks();
+
+    // The controller debounces its resize. Without fake timers a trailing
+    // resize outlives the test that scheduled it and runs against mocks that
+    // have since been cleared, in whatever test file happens to be running.
+    vi.useFakeTimers();
+  });
+  afterEach(() => {
+    vi.useRealTimers();
   });
 
   const configWithAspectRatio: CameraDimensionsConfig = {
@@ -622,13 +630,6 @@ describe('MediaDimensionsContainerController', () => {
   });
 
   describe('should respond to slot changes', () => {
-    beforeEach(() => {
-      vi.useFakeTimers();
-    });
-    afterEach(() => {
-      vi.useRealTimers();
-    });
-
     it('should resize container on slotchange event', () => {
       const host = createLitElement();
       host.getBoundingClientRect = vi.fn().mockReturnValue({
@@ -663,13 +664,6 @@ describe('MediaDimensionsContainerController', () => {
   });
 
   describe('should respond to media load', () => {
-    beforeEach(() => {
-      vi.useFakeTimers();
-    });
-    afterEach(() => {
-      vi.useRealTimers();
-    });
-
     it('should resize container on media load', () => {
       const host = createLitElement();
       host.getBoundingClientRect = vi.fn().mockReturnValue({