Add user login, signup, and session.

app/api/users/login/route.ts

@@ -0,0 +1,11 @@
+const BACKEND_URL = process.env.BACKEND_URL;
+
+export async function POST(request: Request) {
+  const response = await fetch(`${BACKEND_URL}/users/token`, request);
+
+  const data = await response.json();
+
+  return Response.json(data, {
+    status: response.status,
+  });
+}