fix: restore missing middleware file from main

The middleware.ts file was missing from cn/main branch, causing /admin to return 404 instead of redirecting to /admin/users. This restores the middleware from main branch.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: mrjasonroy

src/middleware.ts

@@ -0,0 +1,31 @@
+import { NextResponse, type NextRequest } from "next/server";
+import { getSessionCookie } from "better-auth/cookies";
+
+export async function middleware(request: NextRequest) {
+  const { pathname } = request.nextUrl;
+
+  /*
+   * Playwright starts the dev server and requires a 200 status to
+   * begin the tests, so this ensures that the tests can start
+   */
+  if (pathname.startsWith("/ping")) {
+    return new Response("pong", { status: 200 });
+  }
+
+  if (pathname === "/admin") {
+    return NextResponse.redirect(new URL("/admin/users", request.url));
+  }
+
+  const sessionCookie = getSessionCookie(request);
+
+  if (!sessionCookie) {
+    return NextResponse.redirect(new URL("/sign-in", request.url));
+  }
+  return NextResponse.next();
+}
+
+export const config = {
+  matcher: [
+    "/((?!_next/static|_next/image|favicon.ico|sitemap.xml|robots.txt|api/auth|export|sign-in|sign-up).*)",
+  ],
+};