Huge backend refactor for streamlined styling`

[jannisborn]

Let's use isort, black and ruff everywhere.

• sorted all files in backend with isort
• formatted all files in backend with black
• formatted all files in backend with ruff (slightly stricter, e.g., detecting unused imports)
• added pre-commit hooks for all three
• added a CI workflow to check the code after every commit
• added docstrings for all functions
• added static type annotations for all functions
• added exemplary vscode settings that should help formatting the code automatically on save (requires some vscode extensions for black/isort/ruff)
• recommend to auto-generate docstring with autoDocstring extension, let's use google-notypes as docstring format

[NinaWie]

Great effort in documentation / refactoring / formatting!! Thanks a lot!
I left some minor comments / questions for clarification.
Did you test it on the server? There are not so many changes that could break the backend, but would be better to double check