Fix missing indexes.

models/article.js

@@ -49,6 +49,9 @@ let model = db.define('article', {
     {
       fields: ['user_id']
     },
+    {
+      fields: ['problem_id']
+    },
     {
       fields: ['sort_time']
     }

models/judge_state.js

@@ -76,6 +76,9 @@ let model = db.define('judge_state', {
       },
       {
         fields: ['problem_id'],
+      },
+      {
+        fields: ['task_id'],
       }
     ]
   });