refactor: 移除注册验证码，删掉 pillow

验证码只剩注册一处调用方，一并去掉。

- UserRegisterAPI 的校验、UserRegisterSerializer 的 captcha 字段、
  发图的 captcha 路由
- utils/captcha/ 整个包删除，含 Menlo.ttc 和 timesbi.ttf 两个字体
- utils/shortcuts.py 的 img2base64()，唯一调用方是验证码视图
- 依赖删除 pillow（本地实测 PIL/ 5.9M + pillow.libs/ 14M）

注意：注册接口现在只有 SysOptions.allow_register 一个开关，没有限流，
throttling 目前只用在 SubmissionAPI 上。内网自用可以接受，站点若暴露到
公网需要补 IP 维度限流。

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

account/serializers.py

@@ -19,7 +19,6 @@ class UserRegisterSerializer(serializers.Serializer):
     username = serializers.CharField(max_length=32)
     password = serializers.CharField(min_length=6)
     email = serializers.EmailField(max_length=64)
-    captcha = serializers.CharField()
 
 
 class UserChangePasswordSerializer(serializers.Serializer):

account/urls/oj.py

@@ -1,7 +1,5 @@
 from django.urls import path
 
-from utils.captcha.views import CaptchaAPIView
-
 from ..views.oj import (
     SSOAPI,
     AvatarUploadAPI,
@@ -27,7 +25,6 @@ urlpatterns = [
     path("register", UserRegisterAPI.as_view()),
     path("change_password", UserChangePasswordAPI.as_view()),  # DEPRECATED: 前端未调用
     path("change_email", UserChangeEmailAPI.as_view()),  # DEPRECATED: 前端未调用
-    path("captcha", CaptchaAPIView.as_view()),
     path("check_username_or_email", UsernameOrEmailCheck.as_view()),  # DEPRECATED: 前端未调用
     path("profile", UserProfileAPI.as_view(), name="user_profile_api"),
     path("profile/fresh_display_id", ProfileProblemDisplayIDRefreshAPI.as_view()),

account/views/oj.py

@@ -14,7 +14,6 @@ from problem.models import Problem
 from submission.models import JudgeStatus, Submission
 from utils.api import APIView, AsyncAPIView, CSRFExemptAPIView, validate_serializer
 from utils.async_helpers import async_cache_get, async_cache_set
-from utils.captcha import Captcha
 from utils.constants import CacheKey
 from utils.shortcuts import datetime2str, rand_str
 
@@ -159,9 +158,6 @@ class UserRegisterAPI(AsyncAPIView):
         data = request.data
         data["username"] = data["username"].lower()
         data["email"] = data["email"].lower()
-        captcha = Captcha(request)
-        if not captcha.check(data["captcha"]):
-            return self.error("Invalid captcha")
         if await User.objects.filter(username=data["username"]).aexists():
             return self.error("Username already exists")
         if await User.objects.filter(email=data["email"]).aexists():