Override default Action Mailer `preview_path` (#26110)

config/application.rb

@@ -192,7 +192,9 @@ module Mastodon
     # config.autoload_paths += Dir[Rails.root.join('app', 'api', '*')]
 
     config.active_job.queue_adapter = :sidekiq
+
     config.action_mailer.deliver_later_queue_name = 'mailers'
+    config.action_mailer.preview_path = Rails.root.join('spec', 'mailers', 'previews')
 
     # We use our own middleware for this
     config.public_file_server.enabled = false