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/controller/api/v1/user/UserSign.php

@@ -6,6 +6,7 @@
 namespace app\controller\api\v1\user;
 
 use app\Request;
+use app\services\article\ArticleServices;
 use app\services\user\member\MemberCardServices;
 use app\services\user\UserServices;
 use app\services\user\UserSignServices;
@@ -147,4 +148,26 @@ class UserSign
         $this->services->setSignRemind($uid, $status);
         return app('json')->success('设置成功');
     }
+
+    /**
+     * 签到前置阅读：从"签到广告"分类下随机返回一篇文章
+     * 分类不存在 / 无文章 时返回 null，前端跳过门槛直接签到
+     */
+    public function required_article(ArticleServices $articleServices)
+    {
+        $article = $articleServices->getRandomByCategoryTitle('签到广告');
+        if (!$article) {
+            return app('json')->successful(null);
+        }
+        $img = $article['image_input'] ?? '';
+        if (is_array($img)) {
+            $img = $img[0] ?? '';
+        }
+        return app('json')->successful([
+            'id'                    => (int)($article['id'] ?? 0),
+            'title'                 => (string)($article['title'] ?? ''),
+            'image'                 => (string)$img,
+            'view_required_seconds' => 10,
+        ]);
+    }
 }