fix: make production compose portable

[technowizard]

Summary

• Make the production web image portable by removing build-time VITE_API_URL
• Default web API calls to same-origin and proxy API routes through nginx at runtime
• Add runtime API_UPSTREAM support for the web nginx config
• Make production compose host ports configurable with SERVER_PUBLIC_PORT and WEB_PUBLIC_PORT
• Add .env.prod.example with production-safe placeholders