ci: add windows-latest to test matrix #123

Job	Run time
Frontend unit tests (vitest) (ubuntu-latest)	15s
Verify prod requirements.txt is sufficient	8s
mypy	17s
API integration tests + coverage (windows-latest)	32s
API integration tests + coverage (ubuntu-latest)	14s
pytest (windows-latest)	39s
pytest (ubuntu-latest)	16s
Frontend unit tests (vitest) (windows-latest)	50s
	3m 11s

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