implement suggestions from static code analysis tools

app/controllers/categories_controller.rb

@@ -1,10 +1,24 @@
 # frozen_string_literal: true
 
 class CategoriesController < SqmApplicationController
+  before_action :response_rate_timestamp, only: [:index]
   helper GaugeHelper
+
   def show
     @categories = Category.sorted.map { |category| CategoryPresenter.new(category:) }
 
     @category = CategoryPresenter.new(category: Category.find_by_slug(params[:id]))
   end
+
+  private
+
+  def response_rate_timestamp
+    @response_rate_timestamp = begin
+      rate = ResponseRate.where(school: @school,
+                                academic_year: @academic_year).order(updated_at: :DESC).first || Today.new
+
+      rate.updated_at
+    end
+    @response_rate_timestamp
+  end
 end