fix: convert Web API ReadableStream to Node.js Readable for Fastify

graphql-yoga's handleNodeRequestAndResponse returns a Response with a
Web API ReadableStream body. Fastify's reply.send() requires a Node.js
Readable stream, causing all GraphQL requests to hang indefinitely.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/backend/src/index.ts

@@ -4,6 +4,7 @@ import fastifyCors from "@fastify/cors";
 import fastifyMultipart from "@fastify/multipart";
 import fastifyStatic from "@fastify/static";
 import { createYoga } from "graphql-yoga";
+import { Readable } from "stream";
 import path from "path";
 import { schema } from "./graphql/index";
 import { buildContext } from "./graphql/context";
@@ -69,7 +70,7 @@ async function main() {
       for (const [key, value] of response.headers.entries()) {
         reply.header(key, value);
       }
-      return reply.send(response.body);
+      return reply.send(Readable.from(response.body));
     },
   });