fix(vad): lower activation threshold 0.60→0.50

Threshold 0.60 too strict, user speech consistently not detected.
Back to default 0.50 with min_speech_duration=0.2 as noise guard.

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

voice.py

@@ -85,7 +85,7 @@ def _get_vad():
     global _vad
     if _vad is None:
         _vad = silero.VAD.load(
-            activation_threshold=0.60,
+            activation_threshold=0.50,
             min_speech_duration=0.2,
             min_silence_duration=0.55,
         )