feat: new background work process

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

Procfile.dev

@@ -1,5 +1,3 @@
 web: bundle exec puma -C config/puma.rb
 worker: bundle exec ruby script/worker.rb
-cron: bundle exec rake postal:cron
-smtp: bundle exec rake postal:smtp_server
-requeuer: bundle exec rake postal:requeuer
+smtp: bundle exec ruby script/smtp_server.rb