feat(api): add sign required-article endpoint

GET /api/sign/required_article returns one random article from the
"签到广告" article category (status=1, is_del=0, hidden=0). Empty
category or no articles returns null so clients can fall through to
direct sign-in. Powers the pre-sign-in reading gate per
docs/project-shaoyaoju/prd-require.md §6.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pro_v3.5.1/app/services/article/ArticleServices.php

@@ -119,6 +119,37 @@ class ArticleServices extends BaseServices
         return $this->dao->update($id, ['product_id' => $product_id]);
     }
 
+    /**
+     * 按分类标题获取一条随机文章（用于签到前置阅读门槛）
+     * @param string $categoryTitle 分类标题（如"签到广告"）
+     * @return array|null id/title/image_input/visit；分类或文章不存在时返回 null
+     */
+    public function getRandomByCategoryTitle(string $categoryTitle)
+    {
+        /** @var ArticleCategoryServices $categoryServices */
+        $categoryServices = app()->make(ArticleCategoryServices::class);
+        $cat = $categoryServices->getOne([
+            'title'  => $categoryTitle,
+            'status' => 1,
+            'is_del' => 0,
+            'hidden' => 0,
+        ], 'id');
+        if (!$cat) {
+            return null;
+        }
+        $where = ['cid' => (int)$cat['id'], 'status' => 1, 'hide' => 0];
+        $ids = $this->dao->getColumn($where, 'id');
+        if (!$ids) {
+            return null;
+        }
+        $pickId = (int)$ids[array_rand($ids)];
+        $article = $this->dao->getOne(['id' => $pickId], 'id,title,image_input,visit,add_time');
+        if (!$article) {
+            return null;
+        }
+        return is_array($article) ? $article : $article->toArray();
+    }
+
     /**
      * 获取数量
      * @param array $where