Replace the blanket tg.cancel_scope.cancel() with a per-task CancelScope: the
readiness loop reports its scope via task_status, and the lifespan cancels just
that scope at shutdown. The task group's exit then waits for the scanner/
processor tasks to drain on their shutdown_event instead of force-cancelling
them mid-work, restoring the graceful drain the pre-refactor code had.
Also use docstrings instead of bare `return` in the no-op mode closures.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
