ci: add SAST workflows (semgrep + bandit) with dep-pinned installs

[louiseschmidtgen]

Summary

Add static application security testing (SAST) workflows with hash-pinned pip dependencies, per the security team's recommendation that every repository implement SAST scanning.

Changes

Semgrep SAST workflow (.github/workflows/semgrep.yaml)

• Runs p/python and p/github-actions rulesets on push/PR
• Advisory-only: results uploaded to GitHub Security tab via SARIF

Bandit security scan (.github/workflows/bandit.yaml)

• Scans Python code for common security anti-patterns
• Blocks on HIGH severity only (-lll flag)
• Runs on push/PR and weekly (Monday 09:00 UTC)

Dependency pinning (ci/requirements-*.{in,txt})

• Pin semgrep and bandit pip installs with hash verification
• Satisfies OSSF Scorecard Pinned-Dependencies checks for pip

Reference

• Semgrep: ci: add Semgrep SAST scanning workflow k8s-snap#2468
• Bandit: ci: add Bandit SAST security scanning k8s-snap#2466
• Dep pinning: ci: pin pip dependencies with hash verification k8s-snap#2492

[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.