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/config.py

@@ -33,7 +33,7 @@ class PerformanceSettings(BaseModel):
 class EmbedderSettings(BaseModel):
     """CLIP / SigLIP embedding model settings.
     Supported: "openclip_vitb32" (512-d), "siglip2_vitb16" (768-d, default)."""
-    name: str = "siglip2_vitb16"
+    name: str = "openclip_vitb32"
     batch_size: int = 8
 
 class OCRSettings(BaseModel):