fix: vision tasks inherit user_id, admin owns mount root

- detect_objects, classify_content, recluster_faces now look up the
  photo's user_id and set it on created Tag rows — fixes tags being
  invisible to the owning user due to NULL user_id
- Initial admin setup creates source root at the mount root (/photos)
  instead of a subdirectory, since the admin owns the entire library
- Revert to OpenCLIP ViT-B/32 (512-d) as default embedder — SigLIP
  requires transformers version alignment not yet available in the
  Docker image. SigLIP2 code remains for future enablement.
- Add transformers to requirements for future SigLIP support

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

backend/requirements.txt

@@ -37,6 +37,7 @@ watchfiles==0.21.0
 # Vision pipeline (ONNX Runtime CPU inference)
 onnxruntime==1.18.1
 open-clip-torch==2.24.0     # tokenizer + export helper; inference via ONNX
+transformers>=4.37.0        # HuggingFace tokenizer for SigLIP models
 ultralytics==8.4.37          # YOLOv8n export helper; inference via ONNX
 rapidocr-onnxruntime==1.3.22
 scikit-learn==1.4.0          # DBSCAN for face clustering