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/call-in.html

@@ -150,6 +150,6 @@
         </div>
     </div>
 
-    <script src="/js/call-in.js?v=3"></script>
+    <script src="/js/call-in.js?v=4"></script>
 </body>
 </html>