feat: raise default scan threshold from 0.30 to 0.50

Calibrated classifiers output true probabilities, so 0.50 is the
natural decision boundary.

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

core/audio_scan.py

@@ -737,7 +737,7 @@ def prefetch_audio(video_path: str, embed_model: str | None = None,
 def scan_video(
     video_path: str,
     model: dict = None,
-    threshold: float = 0.30,
+    threshold: float = 0.50,
     hop: float = 1.0,
     window: float = _WINDOW,
     cancel_flag: object = None,