Increase BROADCAST_CAPACITY to fix client lag warnings

The broadcast channel capacity was too small (512) causing clients
to lag behind the encoder and drop thousands of messages, resulting
in choppy playback. Increased to 4096 to provide ~5 minutes of
buffer at typical FLAC streaming rates (~12-15 chunks/sec at 8KB each).

This resolves the "FLAC client lagged, skipped N messages" warnings.

pmoaudio-ext/src/sinks/streaming_flac_sink.rs

@@ -81,7 +81,9 @@ use tracing::{debug, error, info, trace, warn};
 const DEFAULT_ICY_METAINT: usize = 16000;
 
 /// Broadcast channel capacity for FLAC bytes.
-const BROADCAST_CAPACITY: usize = 512;
+/// Increased to 4096 to handle network backpressure and late-joining clients.
+/// At ~12-15 chunks/sec (8KB each), this provides ~5 minutes of buffer.
+const BROADCAST_CAPACITY: usize = 4096;
 
 /// Snapshot of track metadata at a point in time.
 ///