Record Google Scholar citation fetch failures

[systemreliability]

Summary

• Preserve existing citation counts when Google Scholar fetch fails
• Record fetch_status, fetch_failed_at, and fetch_error in _data/citations.yml on fetch failure
• Keep the workflow red after committing failure metadata so Scholar outages are visible instead of silently passing
• Clear failure metadata on the next successful fetch

Verification

• Ran python3 -m py_compile bin/update_scholar_citations.py
• Ran git diff --check

Notes

The investigated failure was caused by scholarly being unable to fetch Google Scholar from the GitHub Actions runner after 3 retries, not by push permissions.