Code Review Summary

This PR addresses CORS preflight failures for peer deployment requests by whitelisting the custom X-FlatRun-Deployment header. It also introduces a unit test to ensure this configuration remains functional.

🚀 Key Improvements

• Whitelisted X-FlatRun-Deployment in the API's CORS configuration to support cross-origin peer requests.
• Added internal/api/cors_test.go to validate that the CORS middleware correctly handles preflight requests containing the new custom header.

💡 Minor Suggestions

• Use a constant for the custom header name to ensure consistency between the configuration and the handlers that consume it.