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/export_models.py

@@ -135,9 +135,9 @@ def export_siglip2(models_dir: Path):
         logger.info("SigLIP2 ONNX files already exist, skipping export")
         return
 
-    logger.info("Loading SigLIP2 ViT-B-16-SigLIP2 webli...")
+    logger.info("Loading SigLIP2 ViT-B-16-SigLIP-384 webli...")
     model, _, preprocess = open_clip.create_model_and_transforms(
-        "ViT-B-16-SigLIP2", pretrained="webli"
+        "ViT-B-16-SigLIP-384", pretrained="webli"
     )
     model.eval()
 
@@ -162,7 +162,7 @@ def export_siglip2(models_dir: Path):
     # ── Textual encoder ───────────────────────────────────────────────
     if not textual_path.exists():
         logger.info("Exporting SigLIP2 textual encoder → %s", textual_path)
-        tokenizer = open_clip.get_tokenizer("ViT-B-16-SigLIP2")
+        tokenizer = open_clip.get_tokenizer("ViT-B-16-SigLIP-384")
         dummy_text = tokenizer(["a photo"]).to(torch.int64)
 
         class SigLIP2TextEncoder(torch.nn.Module):