Add dependency security maintenance

[tnotheis]

Readiness checklist

• I ensured that the PR title is good enough for the changelog.
• I labeled the PR.
• I added tests (if necessary).
• I self-reviewed the PR.

Summary

• add a scheduled dependency security maintenance workflow without replacing the reusable workflow
• add an .nsprc config file for better-npm-audit exception management

Template

Mirrors js-soft/ts-serval@3b61a40, using a non-conflicting scheduled workflow filename because this repository owns the reusable workflow at .github/workflows/dependency-security-maintenance.yml.