Improved backend maintainability

[irm-codebase]

Fixes #624

Summary of changes in this pull request

• The backend is now a module that shares only what is necessary to model.py.
• Backend fetching is now a match-case function. We can add new backends as needed.
• Invalid backend requests now return a clear error.

Reviewer checklist

• [ ] Test(s) added to cover contribution
• [ ] Documentation updated
• [ ] Changelog updated
• [ ] Coverage maintained or improved