feat: new background work process

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

app/models/server.rb

@@ -206,7 +206,7 @@ class Server < ApplicationRecord
   end
 
   def queue_size
-    @queue_size ||= queued_messages.retriable.count
+    @queue_size ||= queued_messages.ready.count
   end
 
   def stats
@@ -222,7 +222,7 @@ class Server < ApplicationRecord
 
   # Return the domain which can be used to authenticate emails sent from the given e-mail address.
   #
-  #  @param address [String] an e-mail address
+  # @param address [String] an e-mail address
   # @return [Domain, nil] the domain to use for authentication
   def authenticated_domain_for_address(address)
     return nil if address.blank?