feat(infrastructure): Enhance router and API endpoint with proper error handling

src/Infrastructure/Router.php

@@ -19,11 +19,12 @@ class Router
     {
         foreach ($this->routes as $route) {
             if ($route['method'] === $method && $route['path'] === $uri) {
-                return call_user_func($route['handler']);
+                return \call_user_func($route['handler']);
             }
         }
 
         http_response_code(404);
-        echo "Not Found";
+        header('Content-Type: application/json');
+        echo json_encode(['error' => 'Not Found']);
     }
 }