chore: implement production rate limiting at provider/WAF level before traffic reaches Next.js

[coderabbitai]

The current rate limiting in src/proxy.ts is process-local and not shared across server instances, making it ineffective under horizontal scaling.

File: src/proxy.ts line 9

Action required:
Configure rate limiting at the infrastructure level (e.g., Vercel Edge, Cloudflare WAF, or similar) to protect against abuse before requests reach the Next.js application layer.

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Requested by @Producdevity — related to PR #375