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/embed.py

@@ -129,7 +129,7 @@ class SigLIP2Embedder(Embedder):
 
     def embed_text(self, text: str) -> np.ndarray:
         import open_clip
-        tokenizer = open_clip.get_tokenizer("ViT-B-16-SigLIP2")
+        tokenizer = open_clip.get_tokenizer("ViT-B-16-SigLIP-384")
         tokens = tokenizer([text]).numpy().astype(np.int64)
         inputs = self._textual.get_inputs()
         feed = {inputs[0].name: tokens}