feat(backend): scaffold Fastify+TS + /health

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

backend/src/app.ts

@@ -0,0 +1,7 @@
+import Fastify, { type FastifyInstance } from "fastify";
+import type { AppConfig } from "./config.js";
+export function buildApp(_config: AppConfig): FastifyInstance {
+  const app = Fastify({ logger: false });
+  app.get("/health", async () => ({ status: "ok" }));
+  return app;
+}