refactor: 移除 2FA 功能与三个冗余依赖

2FA 早已是死代码：TOTP 相关端点标注为前端未调用，前端唯一的
two_factor_auth 是硬编码 false，没有任何入口能打开它。

- 删除 TwoFactorAuthAPI、CheckTFARequiredAPI、_totp* 辅助函数、
  TwoFactorAuthCodeSerializer 及各 serializer 的 tfa_code 字段
- 删除管理端写 two_factor_auth/tfa_token 的分支。登录已不再校验
  TOTP，若保留写入路径，置 True 会变成静默的安全降级
- 删除 User.two_factor_auth / tfa_token 字段（迁移 0009）
- 移除 django-dbconn-retry：仅挂在 INSTALLED_APPS，代码零引用。
  DATABASES 未配置 CONN_MAX_AGE（默认 0），本就没有持久连接需要
  重连修复。要开持久连接用 Django 自带的 CONN_HEALTH_CHECKS
- requests 换成 httpx（openai 已经引入）：删除废弃的
  ReleaseNotesAPI，judge dispatcher 显式传 timeout=None 以保持
  requests 原本的无超时行为，判题请求是同步等结果的

依赖净减 5 个：otpauth、qrcode、requests、charset-normalizer、
django-dbconn-retry

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

account/serializers.py

@@ -8,7 +8,6 @@ from .models import AdminType, ProblemPermission, User, UserProfile
 class UserLoginSerializer(serializers.Serializer):
     username = serializers.CharField()
     password = serializers.CharField()
-    tfa_code = serializers.CharField(required=False, allow_blank=True)
 
 
 class UsernameOrEmailCheckSerializer(serializers.Serializer):
@@ -26,13 +25,11 @@ class UserRegisterSerializer(serializers.Serializer):
 class UserChangePasswordSerializer(serializers.Serializer):
     old_password = serializers.CharField()
     new_password = serializers.CharField(min_length=6)
-    tfa_code = serializers.CharField(required=False, allow_blank=True)
 
 
 class UserChangeEmailSerializer(serializers.Serializer):
     password = serializers.CharField()
     new_email = serializers.EmailField(max_length=64)
-    tfa_code = serializers.CharField(required=False, allow_blank=True)
 
 
 class GenerateUserSerializer(serializers.Serializer):
@@ -61,7 +58,6 @@ class UserAdminSerializer(serializers.ModelSerializer):
             "real_name",
             "create_time",
             "last_login",
-            "two_factor_auth",
             "open_api",
             "is_disabled",
             "raw_password",
@@ -89,7 +85,6 @@ class UserSerializer(serializers.ModelSerializer):
             "problem_permission",
             "create_time",
             "last_login",
-            "two_factor_auth",
             "open_api",
             "is_disabled",
             "class_name",
@@ -121,7 +116,6 @@ class EditUserSerializer(serializers.Serializer):
     admin_type = serializers.ChoiceField(choices=AdminType.choices)
     problem_permission = serializers.ChoiceField(choices=ProblemPermission.choices)
     open_api = serializers.BooleanField()
-    two_factor_auth = serializers.BooleanField()
     is_disabled = serializers.BooleanField()
     class_name = serializers.CharField(required=False, allow_null=True, allow_blank=True)
 
@@ -152,10 +146,6 @@ class SSOSerializer(serializers.Serializer):
     token = serializers.CharField()
 
 
-class TwoFactorAuthCodeSerializer(serializers.Serializer):
-    code = serializers.IntegerField()
-
-
 class ImageUploadForm(forms.Form):
     image = forms.FileField()