refactor: migrate middleware.ts to proxy.ts (Next.js 16)

proxy.ts

@@ -1,6 +1,8 @@
 import { neonAuthMiddleware } from "@neondatabase/auth/next/server";
 
-export default neonAuthMiddleware({
+// Next.js 16 renamed middleware to proxy
+// neonAuthMiddleware returns a middleware function, we export it as proxy
+export const proxy = neonAuthMiddleware({
   // Redirects unauthenticated users to sign-in page
   loginUrl: "/auth/sign-in",
 });