fix(vad): lower activation threshold 0.75→0.60

0.75 too strict, user voice not detected. 0.60 with min_speech_duration=0.2
should balance noise rejection vs speech detection.

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

voice.py

@@ -84,7 +84,7 @@ def _get_vad():
     global _vad
     if _vad is None:
         _vad = silero.VAD.load(
-            activation_threshold=0.75,
+            activation_threshold=0.60,
             min_speech_duration=0.2,
             min_silence_duration=0.55,
         )