[infra] Consolidate web API deployment and routing architecture

[Dipraise1]

Problem

The repo contains Next API routes, a FastAPI backend under engram-web/api, Next rewrites to NEXT_PUBLIC_API_URL, and Vercel experimental services. These paths overlap and can point at different backends/default miner URLs.

Impact

Production behavior is hard to reason about and easy to misconfigure.

References

• engram-web/next.config.js
• vercel.json
• engram-web/api/main.py
• engram-web/app/api/**

Acceptance criteria

• Pick one production API path for subnet/web operations.
• Remove or clearly mark deprecated backend paths.
• Align default URLs/env vars.
• Update deployment docs.