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/serializers.py

@@ -45,10 +45,12 @@ class ArticleCommentSerializer(ModelSerializer):
 class ArticleListSerializer(ModelSerializer):
     author_name = CharField(source='author.first_name', read_only=True)
     author_avatar = ImageField(source='author.avatar', read_only=True)
+    author_user_id = serializers.PrimaryKeyRelatedField(source='author', read_only=True)
     comments_count = SerializerMethodField()
     read_time = SerializerMethodField()
     is_hot = SerializerMethodField()
     is_new = SerializerMethodField()
+    is_following = SerializerMethodField()
     publish_date = DateTimeField(source='created_at', format='%Y-%m-%d', read_only=True)
     image = SerializerMethodField()
 
@@ -56,10 +58,18 @@ class ArticleListSerializer(ModelSerializer):
         model = Article
         fields = [
             'id', 'title', 'excerpt', 'category', 'author_name', 'author_avatar',
-            'views', 'likes', 'comments_count', 'publish_date', 'image', 'tags',
+            'author_user_id', 'views', 'likes', 'comments_count', 'publish_date', 'image', 'tags',
             'read_time', 'is_featured', 'is_hot', 'is_new', 'is_top', 'status',
+            'is_following',
         ]
 
+    async def get_is_following(self, obj):
+        request = self.context.get('request')
+        if request and request.user.is_authenticated:
+            from user.models import Follow
+            return await Follow.objects.filter(follower=request.user, following=obj.author_id).aexists()
+        return False
+
     async def get_comments_count(self, obj):
         return await obj.comments.acount()