fix(auth): stop redirecting unauthenticated requests to nonexistent login route

No named 'login' route exists anywhere in this app (it's a pure API
backend for the Nuxt SPA). Authenticate::redirectTo() called
route('login') for any request without an explicit Accept: application/json
header, throwing RouteNotFoundException and surfacing as a 500 instead
of the expected 401.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/Http/Middleware/Authenticate.php

@@ -12,6 +12,6 @@ class Authenticate extends Middleware
      */
     protected function redirectTo(Request $request): ?string
     {
-        return $request->expectsJson() ? null : route('login');
+        return null;
     }
 }