feat(C-06)：关注feed+课程聚合修复

文章列表新增author_user_id/is_following与feed=following过滤；课程students_count改真实在学人数去重统计、新增total_minutes（不可解析不伪造）；author_name走sync_to_async防ASGI同步阻塞。

article/views.py

@@ -77,6 +77,17 @@ class ArticleListCreateView(generics.ListCreateAPIView):
                 Q(tags__overlap=[search])
             )
 
+        # 关注 feed：仅返回当前用户关注作者的文章
+        feed = self.request.query_params.get('feed')
+        if feed == 'following':
+            if not self.request.user.is_authenticated:
+                return Article.objects.none()
+            from user.models import Follow
+            followed_ids = Follow.objects.filter(
+                follower=self.request.user
+            ).values_list('following_id', flat=True)
+            qs = qs.filter(author_id__in=followed_ids)
+
         ordering = self.request.query_params.get('ordering', '-created_at')
         return qs.order_by('-is_top', ordering)
 
@@ -94,6 +105,7 @@ class ArticleListCreateView(generics.ListCreateAPIView):
             openapi.Parameter('category', openapi.IN_QUERY, description='文章分类筛选', type=openapi.TYPE_STRING),
             openapi.Parameter('tag', openapi.IN_QUERY, description='文章标签筛选', type=openapi.TYPE_STRING),
             openapi.Parameter('search', openapi.IN_QUERY, description='搜索关键词（标题、摘要、标签）', type=openapi.TYPE_STRING),
+            openapi.Parameter('feed', openapi.IN_QUERY, description='feed=following 时仅返回当前用户关注作者的文章（需登录）', type=openapi.TYPE_STRING),
             openapi.Parameter('ordering', openapi.IN_QUERY, description='排序方式，默认 -created_at', type=openapi.TYPE_STRING),
             openapi.Parameter('page', openapi.IN_QUERY, description='页码', type=openapi.TYPE_INTEGER),
             openapi.Parameter('page_size', openapi.IN_QUERY, description='每页数量，默认10，最大100', type=openapi.TYPE_INTEGER),