Fix stale media refs after in-place updates

[Shayanide]

Summary

• refresh cached local-media references when file bytes change in place
• reuse each submission's SHA-256 fingerprint for job naming and media resolution
• add regression coverage for repeated submissions through one client
• document the user-visible fix in the changelog

Testing

• uv run ruff check .
• uv run ruff format --check .
• uv run mypy
• uv run pytest --cov=humanevals --cov-report=term-missing (129 passed, 97% coverage)
• uv build
• uvx twine check dist/*