k6 Test Maintenance

Maintain, fix, and improve existing k6 test scripts. Five maintenance tasks, each with a step-by-step procedure in references/workflows.md:

1. Threshold tightening -- adjust threshold values based on observed metrics
2. Version migration -- update scripts for new k6 releases
3. Service change adaptation -- fix tests when the underlying service changes
4. Refactoring -- clean up and modernize test code
5. Best practices audit -- check scripts against current k6 best practices

Core principle: behavior-aware change control

Classify every proposed change by whether it alters the test's runtime behavior: