feat(userbot): after a restart the userbot catches up on what it missed while down, from a saved cursor and within bounds

backend/src/userbot/modules/catchup/__init__.py

@@ -0,0 +1,3 @@
+from userbot.modules.catchup.service import SavedState, catch_up, read_saved_state
+
+__all__ = ["SavedState", "catch_up", "read_saved_state"]