Preserve Shodan CVSS severity

[Mirochill]

Summary

• preserve Shodan cvss data as cvss2.base_score when creating service vulnerabilities
• derive severity from the Shodan CVSS score when no explicit severity is provided
• add a parser regression for Shodan CVSS/severity handling

Fixes #38

Validation

• git diff --check
• git diff --cached --check
• git show --check --stat --oneline HEAD
• rg inspection of Shodan CVSS/severity paths
• Not run locally