Add security workflow and Dependabot configuration

[douglaz]

Summary

• Add comprehensive security scanning workflow
• Configure Dependabot for automated dependency updates

Changes

• Security Workflow (.github/workflows/security.yml):

  • cargo-audit: Scans for known security vulnerabilities in dependencies
  • cargo-deny: Performs supply chain security checks for licenses and banned dependencies
  • Runs on push, pull requests, and weekly schedule (Sundays at 6 AM UTC)
  • Generates and stores audit reports as artifacts

• Dependabot Configuration (.github/dependabot.yml):

  • Monitors Cargo dependencies for updates
  • Monitors GitHub Actions for updates
  • Weekly update schedule (Mondays at 6 AM UTC)
  • Automatic PR creation with proper labeling and commit prefixes

Benefits

• Proactive security vulnerability detection
• Automated dependency management
• Supply chain security validation
• Consistent security scanning schedule

Test Plan

• Workflow syntax validated
• Security workflow will run on merge
• Dependabot will create PRs starting next Monday