ci: add windows-latest to test matrix #126

Job	Run time
API integration tests + coverage (ubuntu-latest)	16s
API integration tests + coverage (windows-latest)	31s
mypy	17s
pytest (windows-latest)	37s
Verify prod requirements.txt is sufficient	11s
pytest (ubuntu-latest)	14s
Frontend unit tests (vitest) (ubuntu-latest)	16s
Frontend unit tests (vitest) (windows-latest)	1m 34s
	3m 56s

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