Add recorded_at to recording serializer (#4929)

app/serializers/recording_serializer.rb

@@ -17,7 +17,7 @@
 # frozen_string_literal: true
 
 class RecordingSerializer < ApplicationSerializer
-  attributes :id, :record_id, :name, :length, :participants, :visibility, :protectable, :created_at
+  attributes :id, :record_id, :name, :length, :participants, :visibility, :protectable, :created_at, :recorded_at
 
   has_many :formats
 end