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/classify.py

@@ -63,7 +63,7 @@ class CLIPContentClassifier(ContentClassifier):
         # image embeddings.
         embedder_name = settings.embedder.name
         if embedder_name.startswith("siglip2"):
-            model_arch = "ViT-B-16-SigLIP2"
+            model_arch = "ViT-B-16-SigLIP-384"
             pretrained = "webli"
         else:
             model_arch = "ViT-B-32"