fix: auto-inject frontend NEXT_PUBLIC_API_URL from backend service

[Ho1yShif]

What

Wires the frontend static site's NEXT_PUBLIC_API_URL to the backend's public URL automatically, instead of requiring a manual value set after the first deploy.

- key: NEXT_PUBLIC_API_URL
  fromService:
    name: pydantic-agents-workflows-api
    type: web
    envVarKey: RENDER_EXTERNAL_URL

Why

The public onrender.com URL isn't a first-class fromService property (only host, port, hostport, connectionString are). But fromService also supports envVarKey, which pulls any env var off another service — including Render's auto-injected RENDER_EXTERNAL_URL (https://<slug>-XXXX.onrender.com).

Benefits:

• No manual post-deploy step — the value is resolved automatically.
• Handles the unpredictable -XXXX slug suffix — pulls the real assigned value rather than guessing ${slug}.onrender.com.
• Works at build time — fromService resolves before the static site builds, so NEXT_PUBLIC_API_URL is correctly baked into the bundle.

No cyclic dependency

The dependency is one-directional (frontend → backend). The backend defaults CORS to ["*"] (backend/config.py) and never references the frontend's URL, so there's no mutual fromService reference that could error on apply.

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