Add rate limiting for notifications

**Feature request**
It would be great to have backup for types.

**Use case**
Many users need this functionality for production deployments.

**Implementation idea**
Add a new config option `enable_backup` with default `false`.

The issue names no files, tests, or entry points. First resolve whether the request is notification rate limiting or backup support, then locate the notification and configuration entry points and check how options are tested. Done should include an agreed scope, the new behavior, and coverage for the default-disabled setting.

Written by the indexing model from the issue text.