added api logic, vitest, minimal testing ui

api/src/plugins/request-id.ts

@@ -0,0 +1,15 @@
+// api/src/plugins/request-id.ts
+import type { FastifyPluginCallback } from "fastify";
+import { randomUUID } from "crypto";
+
+const requestIdPlugin: FastifyPluginCallback = (app, _opts, done) => {
+  app.addHook("onRequest", async (req, reply) => {
+    const incoming = (req.headers["x-request-id"] as string | undefined)?.trim();
+    const id = incoming && incoming.length > 0 ? incoming : randomUUID();
+    (req as any).id = id;               // attach to request
+    reply.header("x-request-id", id);    // echo on response
+  });
+  done();
+};
+
+export default requestIdPlugin;