Optimize cross-search CSV export

[karilint]

Closes #908

Summary

• skip the full_count window-count column when fetching cross-search export batches
• stop mutating export rows to remove full_count
• only normalize string line breaks when a field actually contains line breaks

Validation

• npm run lint:backend
• npm run tsc:backend
• cd backend && DOTENV_CONFIG_PATH=../.test.env PORT=4000 node -r dotenv/config ./node_modules/.bin/jest src/api-tests/crossSearch/export.test.ts --runInBand --config jest-config.js --detectOpenHandles --forceExit
• pre-commit hook: npm run lint and npm run tsc