FEATURE: Add category_id to TopicViewWordpressSerializer (#10372)

app/serializers/topic_view_wordpress_serializer.rb

@@ -4,6 +4,7 @@ class TopicViewWordpressSerializer < ApplicationSerializer
 
   # These attributes will be delegated to the topic
   attributes :id,
+             :category_id,
              :posts_count,
              :filtered_posts_count,
              :posts
@@ -15,6 +16,10 @@ class TopicViewWordpressSerializer < ApplicationSerializer
     object.topic.id
   end
 
+  def category_id
+    object.topic.category_id
+  end
+
   def posts_count
     object.topic.posts_count
   end