Add TypeScript types for lib

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

**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 the lib, middleware, and a new enable_backup configuration option, but no files or tests. Start by locating the middleware configuration and existing option tests, then verify how defaults are handled. Done means the option is available with a false default and backup middleware behavior is covered by tests.

Written by the indexing model from the issue text.