ci: add Bandit + CodeQL security scan workflow

[Mangesh-Bhattacharya]

Adds a real GitHub Actions workflow that runs Bandit SAST and CodeQL on every push/PR to main. Verified locally: Bandit reports 0 medium/high findings on the current codebase (10 low-severity, non-blocking). Also adds a genuine build-status badge to the README (replacing having none) and a license badge.

Note: GitHub Dependabot flagged 4 vulnerabilities (3 moderate, 1 low) in dependencies on this push -- worth reviewing separately at /security/dependabot.

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

What Enabling Code Scanning Means:

• The 'Security' tab will display more code scanning analysis results (e.g., for the default branch).
• Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results.
• You will be able to see the analysis results for the pull request's branch on this overview once the scans have completed and the checks have passed.

For more information about GitHub Code Scanning, check out the documentation.

[Mangesh-Bhattacharya]

Closing this in favor of #6 -- found an existing 'portfolio-audit-fixes' branch/PR from an earlier session that covered overlapping ground. Everything unique from this branch (security fixes, CI config) has been folded into #6 instead of duplicating effort.