Reduce live caller latency and improve reliability

- Replace per-callback async task spawning with persistent queue-based sender
- Buffer host mic to 60ms chunks (was 21ms) to reduce WebSocket frame rate
- Reduce server ring buffer prebuffer from 150ms to 80ms
- Reduce browser playback jitter buffer from 150ms to 100ms

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/js/call-in.js

@@ -84,7 +84,7 @@ class PlaybackProcessor extends AudioWorkletProcessor {
         this.readPos = 0;
         this.available = 0;
         this.started = false;
-        this.jitterMs = 150; // buffer 150ms before starting playback
+        this.jitterMs = 100; // buffer 100ms before starting playback
         this.jitterSamples = Math.floor(16000 * this.jitterMs / 1000);
 
         this.port.onmessage = (e) => {