fix(export): surface bulk partial failures via X-Export-Warnings header #140

Job	Run time
pytest (ubuntu-latest)	19s
Frontend unit tests (vitest) (ubuntu-latest)	15s
ruff + pip-audit (ubuntu-latest)	24s
API integration tests + coverage (windows-latest)	45s
mypy	25s
pytest (windows-latest)	1m 20s
API integration tests + coverage (ubuntu-latest)	16s
Frontend unit tests (vitest) (windows-latest)	43s
ruff + pip-audit (windows-latest)	58s
Verify prod requirements.txt is sufficient	11s
	5m 36s

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