fix: reduce audio jitter buffer latency (#673)

* fix: reduce audio jitter buffer latency

- Release listenersMu before dispatching samples to listeners
- Buffer Track.sample channel (size 2) with non-blocking WriteSample
- Buffer pipeline.sample channel (size 4) to unblock GStreamer thread
- Add leaky=downstream max-size-buffers=5 to audio queue to prevent burst accumulation
- Set sync=false on appsink to deliver buffers immediately without clock wait

These changes break the backpressure chain that caused the GStreamer
streaming thread to stall. When the Go scheduler has a hiccup, buffered
frames were delivered in a burst, causing the browser audio jitter buffer
to grow to ~10 seconds over time.

Fixes: #598

* fix: log message when dropping samples due to full track channel

server/internal/capture/manager.go

@@ -135,7 +135,7 @@ func New(desktop types.DesktopManager, config *config.Capture) *CaptureManagerCt
 				"pulsesrc device=%s "+
 					"! audio/x-raw,channels=2 "+
 					"! audioconvert "+
-					"! queue "+
+					"! queue max-size-buffers=5 leaky=downstream "+
 					"! %s "+
 					"! appsink name=appsink", config.AudioDevice, config.AudioCodec.Pipeline,
 			), nil

server/internal/capture/streamsink.go

@@ -369,7 +369,6 @@ func (manager *StreamSinkManagerCtx) saveSampleBitrate(timestamp time.Time, delt
 
 func (manager *StreamSinkManagerCtx) onSample(sample types.Sample) {
 	manager.listenersMu.Lock()
-	defer manager.listenersMu.Unlock()
 
 	// save to metrics
 	length := float64(sample.Length)
@@ -386,7 +385,14 @@ func (manager *StreamSinkManagerCtx) onSample(sample types.Sample) {
 		manager.listenersKf = make(map[uintptr]types.SampleListener)
 	}
 
+	// copy listeners before releasing lock to avoid holding it during dispatch
+	listeners := make([]types.SampleListener, 0, len(manager.listeners))
 	for _, l := range manager.listeners {
+		listeners = append(listeners, l)
+	}
+	manager.listenersMu.Unlock()
+
+	for _, l := range listeners {
 		l.WriteSample(sample)
 	}
 }