perf: sync protocol and core hardening updates

deploy/migrations/0064_user_update_retention.down.sql

@@ -0,0 +1,20 @@
+-- TDesktop has no account-level differenceTooLong fallback.  If any confirmed prefix has already
+-- been deleted, removing this floor/observed state would turn a durable history hole into a false
+-- empty difference.  A rollback is safe only before retention has advanced anywhere.
+DO $$
+BEGIN
+    IF EXISTS (
+        SELECT 1
+        FROM public.user_update_retention
+        WHERE retained_through_pts > 0
+    ) THEN
+        RAISE EXCEPTION
+            'cannot roll back user update retention: retained floor has advanced'
+            USING ERRCODE = '55000';
+    END IF;
+END
+$$;
+
+DROP INDEX IF EXISTS public.user_update_events_retention_global_idx;
+DROP TABLE IF EXISTS public.user_update_retention;
+ALTER TABLE public.update_states DROP COLUMN IF EXISTS observed_pts;