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.

test/mix/tasks/pleroma/database_test.exs

@@ -424,6 +424,7 @@ defmodule Mix.Tasks.Pleroma.DatabaseTest do
                ["push_subscriptions"],
                ["registrations"],
                ["report_notes"],
+               ["retention_cursors"],
                ["rich_media_card"],
                ["rules"],
                ["scheduled_activities"],