feat: update API endpoints for company-vendor architecture

- Add transfer-ownership endpoint to companies API
- Add user search endpoint for autocomplete (/admin/users/search)
- Update company detail endpoint to include owner info and vendors list
- Update vendor endpoints to use company relationship for ownership
- Update deps.py vendor access check to use company.owner_user_id

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/api/deps.py

@@ -513,13 +513,20 @@ def get_user_vendor(
         VendorNotFoundException: If vendor doesn't exist
         UnauthorizedVendorAccessException: If user doesn't have access
     """
-    vendor = db.query(Vendor).filter(Vendor.vendor_code == vendor_code.upper()).first()
+    from sqlalchemy.orm import joinedload
+
+    vendor = (
+        db.query(Vendor)
+        .options(joinedload(Vendor.company))
+        .filter(Vendor.vendor_code == vendor_code.upper())
+        .first()
+    )
 
     if not vendor:
         raise VendorNotFoundException(vendor_code)
 
-    # Check if user owns this vendor
-    if vendor.owner_user_id == current_user.id:
+    # Check if user owns this vendor (via company ownership)
+    if vendor.company and vendor.company.owner_user_id == current_user.id:
         return vendor
 
     # Check if user is team member