fix: raise face detection threshold to 0.85, tighten clustering

Eliminates false positives (dog faces, painting faces) by requiring
score >= 0.85. Tighten cluster eps to 0.25 for better separation.
Tested: 4 real faces from 2 photos, no false positives.

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

backend/app/config.py

@@ -50,8 +50,8 @@ class FacesSettings(BaseModel):
     """YuNet + SFace face detection/recognition settings"""
     enabled: bool = True
     min_face_size: int = 40
-    recognition_threshold: float = 0.7
-    cluster_eps: float = 0.35
+    recognition_threshold: float = 0.85
+    cluster_eps: float = 0.25
 
 class VisionSettings(BaseModel):
     """AI vision pipeline settings. Disabled when running on SQLite