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/app/services/vision/bootstrap_models.py

@@ -29,8 +29,6 @@ DOWNLOADS = []
 EXPORTS = [
     ("embed/visual.onnx", "OpenCLIP ViT-B/32 visual encoder"),
     ("embed/textual.onnx", "OpenCLIP ViT-B/32 textual encoder"),
-    ("embed_siglip2/visual.onnx", "SigLIP2 ViT-B/16 visual encoder"),
-    ("embed_siglip2/textual.onnx", "SigLIP2 ViT-B/16 textual encoder"),
     ("detect/yolov8n.onnx", "YOLOv8n object detector"),
 ]