ci: add Trivy security scan workflow

[audunru]

Summary

• Adds a Trivy security scan workflow that runs on push to main, pull requests targeting main, and on a weekly schedule (Mondays 00:00 UTC)
• Filesystem scan posts results to GitHub Security tab under category filesystem-scan
• Only CRITICAL and HIGH severity findings reported; unfixed vulnerabilities excluded
• No image-scan job — this is a pure Composer library with no Dockerfile

Test plan

• Filesystem & IaC Scan job completes successfully
• Results appear in the GitHub Security → Code scanning tab

[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.

[audunru]

🎉 This PR is included in version 4.1.0 🎉

The release is available on GitHub release

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