PERF: Fix N+1 queries on /categories route (#19585)

Featured topics are eventually serialized by `ListableTopicSerializer`
which calls `Topic#image_url` which requires us to preload
`Topic#topic_thumbnails`.

app/models/category_list.rb

@@ -62,7 +62,13 @@ class CategoryList
 
     category_featured_topics = CategoryFeaturedTopic.select([:category_id, :topic_id]).order(:rank)
 
-    @all_topics = Topic.where(id: category_featured_topics.map(&:topic_id)).includes(:shared_draft, :category)
+    @all_topics = Topic
+      .where(id: category_featured_topics.map(&:topic_id))
+      .includes(
+        :shared_draft,
+        :category,
+        { topic_thumbnails: [:optimized_image, :upload] }
+      )
 
     @all_topics = @all_topics.joins(:tags).where(tags: { name: @options[:tag] }) if @options[:tag].present?
 
@@ -140,6 +146,7 @@ class CategoryList
     end
 
     allowed_topic_create = Set.new(Category.topic_create_allowed(@guardian).pluck(:id))
+
     categories_with_descendants.each do |category|
       category.notification_level = notification_levels[category.id] || default_notification_level
       category.permission = CategoryGroup.permission_types[:full] if allowed_topic_create.include?(category.id)

spec/requests/categories_controller_spec.rb

@@ -302,10 +302,12 @@ RSpec.describe CategoriesController do
       expect(category_response["subcategory_list"][0]["id"]).to eq(subcategory.id)
     end
 
-    it "does not n+1 with multiple topics" do
+    it "does not result in N+1 queries problem with multiple topics" do
       category1 = Fabricate(:category)
       category2 = Fabricate(:category)
+      upload = Fabricate(:upload)
       topic1 = Fabricate(:topic, category: category1)
+      topic2 = Fabricate(:topic, category: category1, image_upload: upload)
 
       CategoryFeaturedTopic.feature_topics
       SiteSetting.desktop_category_page_style = "categories_with_featured_topics"
@@ -320,9 +322,10 @@ RSpec.describe CategoriesController do
       end
 
       category_response = response.parsed_body["category_list"]["categories"].find { |c| c["id"] == category1.id }
-      expect(category_response["topics"].count).to eq(1)
+      expect(category_response["topics"].count).to eq(2)
 
-      topic2 = Fabricate(:topic, category: category2)
+      upload = Fabricate(:upload)
+      topic3 = Fabricate(:topic, category: category2, image_upload: upload)
       CategoryFeaturedTopic.feature_topics
 
       second_request_queries = track_sql_queries do
@@ -332,7 +335,7 @@ RSpec.describe CategoriesController do
 
       category1_response = response.parsed_body["category_list"]["categories"].find { |c| c["id"] == category1.id }
       category2_response = response.parsed_body["category_list"]["categories"].find { |c| c["id"] == category2.id }
-      expect(category1_response["topics"].size).to eq(1)
+      expect(category1_response["topics"].size).to eq(2)
       expect(category2_response["topics"].size).to eq(1)
 
       expect(first_request_queries.count).to eq(second_request_queries.count)