feat: new background work process

This removes all previous dependencies on RabbitMQ and the need to run separate cron and requeueing processes.

app/scheduled_tasks/expire_held_messages_scheduled_task.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+class ExpireHeldMessagesScheduledTask < ApplicationScheduledTask
+
+  def call
+    Server.all.each do |server|
+      messages = server.message_db.messages(where: {
+        status: "Held",
+        hold_expiry: { less_than: Time.now.to_f }
+      })
+
+      messages.each(&:cancel_hold)
+    end
+  end
+
+end