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[kwonminjae5700]

This pull request adds automated load testing to the CI workflow using k6, targeting the /api/posts/v1/slow endpoint of the FastAPI server. It introduces a new k6 test script and updates the GitHub Actions workflow to install k6, start the FastAPI server, and run the load test as part of the CI pipeline.

CI Workflow Enhancements:

• Added steps to the .github/workflows/main.yml workflow to install k6, start the FastAPI server with uvicorn, and execute the k6 load test against the /api/posts/v1/slow endpoint.
• Minor formatting fix for the python-version key in the workflow YAML.

Load Testing:

• Introduced a new k6-test.js script that defines a k6 load test targeting the /api/posts/v1/slow endpoint, with staged traffic and a threshold on 95th percentile response time.