test: add precheck k6 load test

[Shaivpidadi]

Summary

• add a repo-local k6 load test for
• add a deterministic API key seeding helper for CI/local runs
• extend CI to run the load test and upload summary/raw artifacts

Verification

• black --check app/ tests/
• isort --check-only app/ tests/
• flake8 app/ tests/ --max-line-length=88 --extend-ignore=E203,W503,E501,F401,F811,F841,E402
• mypy app/ --ignore-missing-imports
• pytest tests/ -v --tb=short --cov=app --cov-report=term-missing --cov-fail-under=60
• dockerized k6 run at 100 req/s for 30s: p95=6.82ms, 0.00% HTTP failures