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.

config/description.exs

@@ -3334,8 +3334,10 @@ config :pleroma, :config_description, [
       %{
         key: :batch_size,
         type: :integer,
-        description: "Maximum number of threads to evict per worker run. Default: 500.",
-        suggestions: [500]
+        description:
+          "Number of activities the worker walks per run; their threads are checked and evicted." <>
+            " Default: 50000.",
+        suggestions: [50_000]
       },
       %{
         key: :keep_non_public,