Fix #3189 - Share CSRF secret between main app and Sidekiq Web (#4383)

config/routes.rb

@@ -3,6 +3,8 @@
 require 'sidekiq/web'
 require 'sidekiq-scheduler/web'
 
+Sidekiq::Web.set :session_secret, Rails.application.secrets[:secret_key_base]
+
 Rails.application.routes.draw do
   mount LetterOpenerWeb::Engine, at: 'letter_opener' if Rails.env.development?