feat(retention): walk activities incrementally with a persisted cursor

Grouping the whole activities table by thread every run is unusable on
a large instance: on a 312 GB database it plans as a seq scan plus a
50M-row on-disk sort. Instead, walk activities in id order (flake ids
are time-ordered) from a persisted cursor up to the id that corresponds
to the deadline, and verify only the threads those activities belong to
through the (type, context) index. Measured on that instance: 147 ms
for a 50k-activity range scan, 349 ms warm / 14 s cold to verify ~700
threads.

The cursor is persisted per chunk so a killed run resumes. The
watermark walk keeps a cursor of its own so it cannot leave the
age-based walk stranded behind the deadline. Adds the retention_cursors
table.

priv/repo/migrations/20260902000000_create_retention_cursors.exs

@@ -0,0 +1,16 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Repo.Migrations.CreateRetentionCursors do
+  use Ecto.Migration
+
+  def change do
+    create_if_not_exists table(:retention_cursors, primary_key: false) do
+      add(:name, :string, primary_key: true)
+      add(:position, :uuid, null: false)
+
+      timestamps()
+    end
+  end
+end