feat(chat): add transient presence flash and 5s reconnect grace period

.env.example

@@ -54,8 +54,8 @@ NGINX_PORT=80
 
 # Playout configuration
 PLAYLIST_PATH=/app/playlist/playlist.txt
-HLS_SEGMENT_DURATION=6
-HLS_WINDOW_SIZE=24
+HLS_SEGMENT_DURATION=2
+HLS_WINDOW_SIZE=12
 
 # Shared runtime config (written by chat admin via /api/admin/config, read by playout).
 # Both resolve to the same host file through the ./data/playlist volume.