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/routers/auth.py

@@ -161,7 +161,9 @@ async def setup(body: SetupRequest, db: AsyncSession = Depends(get_db)):
     if len(body.password) < 6:
         raise HTTPException(status_code=400, detail="Password must be at least 6 characters")
 
-    media_path = os.path.join(settings.photo_dirs, body.username.strip())
+    # The initial admin owns the entire photo mount root. Regular users
+    # (created later via admin panel) get a subdirectory under it.
+    media_path = settings.photo_dirs
     os.makedirs(media_path, exist_ok=True)
 
     user = User(
@@ -171,8 +173,8 @@ async def setup(body: SetupRequest, db: AsyncSession = Depends(get_db)):
         media_path=media_path,
     )
     db.add(user)
+    await db.flush()  # get user.id before creating source root
 
-    # Create a source root for the new admin's media directory
     source_root = SourceRoot(
         name=f"{user.username}'s Library",
         path=media_path,