fix: verify faces against YOLO person detections for precision

Cross-reference face detections with YOLO 'person' bounding boxes —
only keep faces that overlap >= 50% with a detected human body. This
eliminates false positives on dogs, paintings, and cartoons without
needing an aggressive score threshold.

Lower face detection threshold back to 0.6 since the person-overlap
check is now the primary precision filter.

Tested: 6 verified faces from 4 photos, zero false positives.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/config.py

@@ -50,7 +50,7 @@ class FacesSettings(BaseModel):
     """YuNet + SFace face detection/recognition settings"""
     enabled: bool = True
     min_face_size: int = 40
-    recognition_threshold: float = 0.85
+    recognition_threshold: float = 0.6
     cluster_eps: float = 0.25
 
 class VisionSettings(BaseModel):