fix(blog): limit category nesting depth

apps/blog/api/views.py

@@ -343,10 +343,14 @@ def _validate_category_parent(category_id: int | None, parent_id: int | None) ->
         return None, None
     if category_id and parent_id == category_id:
         return None, "A category cannot be its own parent."
+    if category_id and Category.objects.filter(parent_id=category_id).exists():
+        return None, "A category with child categories must remain a root category."
 
     parent = Category.objects.filter(id=parent_id).first()
     if not parent:
         return None, "Parent category not found."
+    if parent.parent_id:
+        return None, "Only root categories can be selected as a parent."
 
     current = parent
     seen: set[int] = set()