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/process_message_retention_scheduled_task.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+class ProcessMessageRetentionScheduledTask < ApplicationScheduledTask
+
+  def perform
+    Server.all.each do |server|
+      if server.raw_message_retention_days
+        # If the server has a maximum number of retained raw messages, remove any that are older than this
+        logger.info "Tidying raw messages (by days) for #{server.permalink} (ID: #{server.id}). Keeping #{server.raw_message_retention_days} days."
+        server.message_db.provisioner.remove_raw_tables_older_than(server.raw_message_retention_days)
+      end
+
+      if server.raw_message_retention_size
+        logger.info "Tidying raw messages (by size) for #{server.permalink} (ID: #{server.id}). Keeping #{server.raw_message_retention_size} MB of data."
+        server.message_db.provisioner.remove_raw_tables_until_less_than_size(server.raw_message_retention_size * 1024 * 1024)
+      end
+
+      if server.message_retention_days
+        logger.info "Tidying messages for #{server.permalink} (ID: #{server.id}). Keeping #{server.message_retention_days} days."
+        server.message_db.provisioner.remove_messages(server.message_retention_days)
+      end
+    end
+  end
+
+  def self.next_run_after
+    three_am
+  end
+
+end