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

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

backend/src/server.ts

@@ -0,0 +1,6 @@
+import { buildApp } from "./app.js";
+import { loadConfig } from "./config.js";
+const config = loadConfig(process.env);
+const app = buildApp(config);
+app.listen({ port: config.port, host: "127.0.0.1" })
+  .then((addr) => console.log(`backend listening on ${addr}`));