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.

test/pleroma/workers/cron/retention_worker_test.exs

@@ -0,0 +1,47 @@
+# 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.RetentionWorkerTest do
+  use Pleroma.DataCase, async: false
+
+  alias Pleroma.Object
+  alias Pleroma.Repo
+  alias Pleroma.Web.CommonAPI
+  alias Pleroma.Workers.Cron.RetentionWorker
+
+  import Pleroma.Factory
+
+  setup do
+    clear_config([:instance, :remote_post_retention_days], 90)
+
+    remote_user = insert(:user, local: false)
+    {:ok, post} = CommonAPI.post(remote_user, %{status: "old"})
+
+    old_date =
+      NaiveDateTime.utc_now()
+      |> NaiveDateTime.add(-200 * 86_400)
+      |> NaiveDateTime.truncate(:second)
+
+    post
+    |> Ecto.Changeset.change(%{local: false, updated_at: old_date})
+    |> Repo.update!()
+
+    %{object_id: post.data["object"]}
+  end
+
+  test "does nothing when disabled", %{object_id: object_id} do
+    clear_config([:retention, :enabled], false)
+
+    assert :ok = RetentionWorker.perform(%Oban.Job{})
+    assert Object.get_by_ap_id(object_id)
+  end
+
+  test "evicts when enabled", %{object_id: object_id} do
+    clear_config([:retention, :enabled], true)
+    clear_config([:retention, :batch_size], 500)
+
+    assert :ok = RetentionWorker.perform(%Oban.Job{})
+    refute Object.get_by_ap_id(object_id)
+  end
+end