feat(retention): run eviction from an Oban cron worker

Adds config :pleroma, :retention (disabled by default) and an hourly
crontab entry that evicts one bounded batch per run.

lib/pleroma/workers/cron/retention_worker.ex

@@ -0,0 +1,37 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Workers.Cron.RetentionWorker do
+  @moduledoc """
+  Evicts one batch of old remote threads per run, see `Pleroma.Retention`.
+  """
+
+  use Oban.Worker, queue: :background, max_attempts: 1
+
+  alias Pleroma.Config
+  alias Pleroma.Retention
+
+  require Logger
+
+  @impl true
+  def perform(_job) do
+    config = Config.get(:retention, [])
+
+    if config[:enabled] do
+      stats = Retention.run(config)
+
+      if stats.contexts > 0 do
+        Logger.info(
+          "Retention: evicted #{stats.contexts} threads " <>
+            "(#{stats.objects} objects, #{stats.activities} activities)"
+        )
+      end
+    end
+
+    :ok
+  end
+
+  @impl true
+  def timeout(_job), do: :timer.minutes(30)
+end