Refactor nginx.conf for simplicity, IP forwarding, and high concurrency

[engine-labs-paul]

Summary

Refactored the nginx.conf for improved clarity and correct user IP forwarding in Render.com + Cloudflare setups, while maintaining endpoint proxying and optimizing for high concurrency.

Details

• Reorganized nginx.conf for simplicity, clear comments, and best practices.
• Explicitly added Cloudflare and Render.com trusted proxy IPs with explanations.
• Switched to using X-Forwarded-For and recursive real IP extraction to handle multiple proxy hops correctly.
• Used conditional map logic to always provide most accurate client IP for headers.
• Moved common proxy_set_header lines to the server context for DRYness.
• Maintained Mixpanel endpoint support for /lib.min.js, /lib.js, /decide, and / (catch-all).
• Preserved and enhanced debug/diagnostic header logic.
• Enabled high concurrency settings with worker and server tuning.
• Created a .gitignore file for common log, OS, and editor files.

Warning: Task VM test is not passing, Engine will perform much better if you fix the setup