sounds stupid but would you call hibernate a back end technology or a front end technology?
helps map the table and stored procedures etc into your code, relieved the developer from use of SQL, so where do you think it should be placed?
While many people (unfortunately) perform database access in the presentation tier, but that doesn't make DB access "front end" technology.
But I guess it all comes down to what you define as front-end and back-end. A DB Admin for example, might consider anything that's not the DB itself as "front-end", while a web developer will consider anything not in the presentation tier "back-end".
In my definition every library not designed to display something is not a frontend.
Libraries using services from other libraries are at least middleware.
Libraries providing services are backend.
Just my 2 cents.