fix: face clusters write photo_tags, raise detection threshold to 0.7

- recluster_faces now writes photo_tags rows for each face cluster so
  the tag count and tag_ids filter work (previously count was always 0)
- Old cluster tags and photo_tags are cleaned up before re-clustering
- Raise face detection threshold from 0.4 to 0.7 to reduce false
  positives (was detecting dog faces as people)

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 = 0.4
+    recognition_threshold: float = 0.7
     cluster_eps: float = 0.35
 
 class VisionSettings(BaseModel):