chore: 删 11 个无引用序列化器；撤回一处误判的"修复"

## 撤回：增删题单题目本来就会重算进度，我诊断错了

上一个 commit 我给 ProblemSetProblemAdminAPI 的 post/delete 加了显式的
sync_all_progress_for_problemset，理由是"增删题目不重算进度"。**那是错的。**

problemset/signals.py 里有 post_save / post_delete 信号接收器，
在 ProblemSetProblem 变化时同步进度**并重算奖章**，且 apps.py 的 ready()
确实导入了 signals。我的显式调用只是让同一件事做两遍。

更该记住的是**为什么当时没发现**：那一版测试只看"进度有没有更新"，
而信号和我的调用都会让它更新 —— 分不出是谁干的，所以照样是绿的。
这次的测试改成先撤掉调用再验，并直接断言信号已注册在 post_save/post_delete 上，
这样才真的有区分力。6/6 通过。

同一批修复里的另外 5 条经复核都成立，保留。特别是「移除用户留下孤儿奖章」——
signals.py 里**没有**针对 ProblemSetProgress 的 post_delete，那条必须自己清。

## 删无引用序列化器（-78 行）

全仓 106 个序列化器，11 个无任何引用：

  account:   FileUploadForm, GenerateUserSerializer, SSOSerializer,
             UserChangeEmailSerializer, UserChangePasswordSerializer,
             UsernameOrEmailCheckSerializer
  contest:   CreateContestAnnouncementSerializer, EditContestAnnouncementSerializer
  problem:   CreateProblemCodeTemplateSerializer
  flowchart: FlowchartSubmissionMergedSerializer, FlowchartSubmissionSummarySerializer

前 8 个是上一个 commit 删掉的视图留下的（FileUploadForm 正是
SimditorFileUploadAPIView 用的），后 3 个是原本就死的。
检测器同样做了自检：拿 3 个明确在用的序列化器反测，没有误判。
另外确认这 11 个都没有字符串形式的动态引用。

`ruff check .` 全绿。行尾未被改动（上一轮踩过这个坑，这次脚本里加了断言）。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

account/serializers.py

@@ -10,35 +10,12 @@ class UserLoginSerializer(serializers.Serializer):
     password = serializers.CharField()
 
 
-class UsernameOrEmailCheckSerializer(serializers.Serializer):
-    username = serializers.CharField(required=False)
-    email = serializers.EmailField(required=False)
-
-
 class UserRegisterSerializer(serializers.Serializer):
     username = serializers.CharField(max_length=32)
     password = serializers.CharField(min_length=6)
     email = serializers.EmailField(max_length=64)
 
 
-class UserChangePasswordSerializer(serializers.Serializer):
-    old_password = serializers.CharField()
-    new_password = serializers.CharField(min_length=6)
-
-
-class UserChangeEmailSerializer(serializers.Serializer):
-    password = serializers.CharField()
-    new_email = serializers.EmailField(max_length=64)
-
-
-class GenerateUserSerializer(serializers.Serializer):
-    prefix = serializers.CharField(max_length=16, allow_blank=True)
-    suffix = serializers.CharField(max_length=16, allow_blank=True)
-    number_from = serializers.IntegerField()
-    number_to = serializers.IntegerField()
-    password_length = serializers.IntegerField(max_value=16, default=8)
-
-
 class ImportUserSerializer(serializers.Serializer):
     users = serializers.ListField(child=serializers.ListField(child=serializers.CharField(max_length=64)))
 
@@ -130,18 +107,10 @@ class EditUserProfileSerializer(serializers.Serializer):
     language = serializers.CharField(max_length=32, allow_blank=True, required=False)
 
 
-class SSOSerializer(serializers.Serializer):
-    token = serializers.CharField()
-
-
 class ImageUploadForm(forms.Form):
     image = forms.FileField()
 
 
-class FileUploadForm(forms.Form):
-    file = forms.FileField()
-
-
 class RankInfoSerializer(serializers.ModelSerializer):
     user = UsernameSerializer()