- add a few tests
- re-organize tests to allow testing with different env vars and
creating a combined coverage report
- extract test data into separate files, because importing data from
testA in testB leads to weird jest errors
- remove `dotenv` because it's not being used
- "GitHub Trends" -> "GitHub Stats Extended"
I deleted most workflows from apps/backend/.github/workflows because imo
we don't need them in the near future. I updated and moved two workflows
which we do need. Only the codeQL workflow is left behind, because I
don't have an opinion yet whether it should be removed or kept.