licensing / reuse annotations

atm, everything is licensed under AGPL, as far as I can tell.

For generic / machine generated files like the requirements.txt or the .gitignore, I think AGPL is inadequate. We should use CC0 (public domain) here. Non-source-code files could be licenced CC-BY-4.0-SA (SA to keep the copy-left spirit 😶‍🌫️).

Especially the generic files mentioned above can easily be covered by the REUSE.toml file.