chore: replace jest with vitest in backend project

[marcellmueller]

Description

Besides Vitest being easier to config and more modern than Jest it's a good idea to use Vitest in both the backend and frontend projects for consistency.

Testing this PR

Ensure the testing scripts are working correctly still

cd backend
npm install
npm run test
npm run test:cov
npm run test:e2e

Sonar reporter

I had removed the library jest-sonar-reporter and added vitest-sonar-reporter.

I think this is setup correctly though not 100% sure. Running the tests with jest would create backend/test-report.xml so I configured Vitest to output the same test report file. Let me know if there is something I missed here, though I set the reporter to only run in GitHub actions - see vitest.config.mts.

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[DerekRoberts]

This is great! Thanks a bunch. :)

[marcellmueller]

This is great! Thanks a bunch. :)

Thanks for the quick review! I will get to eslint later this week :)