Refactor audio pipeline for multi-client streaming and improved error handling

Refactor the audio pipeline to support multi-client streaming with new OggFlacStreamHandle and StreamingOggFlacSink.

- Replace DirectOggFlacSink with StreamingOggFlacSink in pipeline
- Update documentation and comments to reflect multi-client support
- Add detailed warning logs for buffer underruns and client disconnections
- Remove TimerBufferNode from pipeline as it's no longer needed
- Update connect() calls to subscribe() for new streaming behavior

This change enables multiple clients to subscribe to the same audio stream, with each subscription getting a live feed from the current point in time.

pmowebrenderer/src/stream.rs

@@ -44,7 +44,7 @@ pub async fn stream_handler(
         }
     };
 
-    let stream = handle.connect().await;
+    let stream = handle.subscribe();
 
     info!(instance_id = %instance_id, "FLAC stream started");