Update index endpoint message

[devin-ai-integration]

Update index endpoint message

Summary

Updated the root endpoint (/) response message from "Hello world!" to "Hello world! This is a simple FastAPI app." to provide a more descriptive message about the application.

This is a simple cosmetic change that doesn't affect any functionality - just makes the root endpoint message more informative.

Review & Testing Checklist for Human

• Verify the updated message is appropriate and professional
• Test that the root endpoint (GET /) still works and returns the expected JSON response

Notes

• This change was made as requested by @samfert-codeium to verify repository access, lint execution, and PR creation capabilities
• All lint checks (flake8, black, mypy) passed successfully
• Link to Devin run: https://app.devin.ai/sessions/88c37a93c8f74927bf1e1961d53745c1

[devin-ai-integration]

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud