feat: new background work process

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

app/models/worker_role.rb

@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+
+# == Schema Information
+#
+# Table name: worker_roles
+#
+#  id          :bigint           not null, primary key
+#  acquired_at :datetime
+#  role        :string(255)
+#  worker      :string(255)
+#
+# Indexes
+#
+#  index_worker_roles_on_role  (role) UNIQUE
+#
+class WorkerRole < ApplicationRecord
+
+  class << self
+
+    # Acquire or renew a lock for the given role.
+    #
+    # @param role [String] The name of the role to acquire
+    # @return [Symbol, false] True if the lock was acquired or renewed, false otherwise
+    def acquire(role)
+      # update our existing lock if we already have one
+      updates = where(role: role, worker: Postal.locker_name).update_all(acquired_at: Time.current)
+      return :renewed if updates.positive?
+
+      # attempt to steal a role from another worker
+      updates = where(role: role).where("acquired_at is null OR acquired_at < ?", 5.minutes.ago)
+                                 .update_all(acquired_at: Time.current, worker: Postal.locker_name)
+      return :stolen if updates.positive?
+
+      # attempt to create a new role for this worker
+      begin
+        create!(role: role, worker: Postal.locker_name, acquired_at: Time.current)
+        :created
+      rescue ActiveRecord::RecordNotUnique, ActiveRecord::RecordInvalid
+        false
+      end
+    end
+
+    # Release a lock for the given role for the current process.
+    #
+    # @param role [String] The name of the role to release
+    # @return [Boolean] True if the lock was released, false otherwise
+    def release(role)
+      updates = where(role: role, worker: Postal.locker_name).delete_all
+      updates.positive?
+    end
+
+  end
+
+end