ci: add security scanning workflow

[jmlweb]

Summary

Adds automated security scanning to the CI pipeline.

What is included

Dependency Audit (pip-audit)

• Scans installed packages for known vulnerabilities
• Uses the PyPI Advisory Database
• Fails on any known vulnerability (--strict mode)

CodeQL Analysis

• Static analysis for Python code
• Detects security vulnerabilities, bugs, and code quality issues
• Results appear in the Security tab on GitHub

When it runs

• ✅ On every push to main
• ✅ On every PR targeting main
• ✅ Weekly (Monday 9:00 UTC) to catch newly disclosed CVEs

Why?

Automated security scanning helps:

• Catch vulnerabilities before they reach production
• Stay informed about new CVEs in dependencies
• Maintain a secure codebase with minimal manual effort

[github-advanced-security]

This pull request sets up GitHub code scanning for this repository. Once the scans have completed and the checks have passed, the analysis results for this pull request branch will appear on this overview. Once you merge this pull request, the 'Security' tab will show more code scanning analysis results (for example, for the default branch). Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results. For more information about GitHub code scanning, check out the documentation.