docs: add FastAPI backend example for non-Node.js setups

[kevin-weitgenant]

What

Adds examples/fastapi-backend/ — a minimal chat app using a FastAPI (Python) backend instead of Next.js.

Why

Every example in the repo uses a Node.js runtime (Next.js, SvelteKit, Nuxt). This example demonstrates that OpenUI is backend-agnostic: the same React frontend works with any server that emits the correct stream format.

The frontend (App.jsx) is identical to the one scaffolded by npx @openuidev/cli create — no changes were needed to switch from Next.js to FastAPI.

What's included

• Backend (~30 lines): FastAPI endpoint that receives { systemPrompt, messages }, calls the OpenAI streaming API, and yields NDJSON chunks
• Frontend: Vite + React app using openAIReadableStreamAdapter() to parse the stream — same as genui-chat-app
• README: Setup instructions and architecture diagram matching the style of other examples

Tested

• Backend streams NDJSON correctly
• Frontend renders OpenUI Lang components in real time
• Verified with gpt-5.2 model

[vishxrad]

minor nit. rest LGTM.

[vishxrad]

please change the model to gpt-5.5. we have a standard model across all examples.