add element descriptions

backend/app/Element/Element.php

@@ -9,6 +9,7 @@ class Element
     public function __construct(
         private int $id,
         private string $title,
+        private string $description,
         private Set $set,
         private ?Element $parentElement,
     ) {
@@ -24,6 +25,11 @@ class Element
         return $this->title;
     }
 
+    public function getDescription(): string
+    {
+        return $this->description;
+    }
+
     public function getSet(): Set
     {
         return $this->set;