精简 Problem 模型：移除 rule_type / total_score / io_mode

系统只用 ACM 模式且题目固定标准 IO，这三列已无实际用途：
- rule_type / total_score：OI 专用，删除后 judge/dispatcher 的 OI 分支全部塌缩为 ACM
- io_mode：判题改为固定发送 Standard IO 常量；languages.py 的 seccomp_rule
  保留 dict 形态但改用字面量 key，不改判题机 wire 契约
- prompt 字段保留（预留给未来 AI 分析）

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

problem/views/admin.py

@@ -19,7 +19,7 @@ from utils.api import APIError, APIView, CSRFExemptAPIView, validate_serializer
 from utils.openai import get_ai_client
 from utils.shortcuts import natural_sort_key, rand_str
 
-from ..models import Problem, ProblemRuleType, ProblemTag
+from ..models import Problem, ProblemTag
 from ..serializers import (
     AddContestProblemSerializer,
     ContestProblemMakePublicSerializer,
@@ -195,14 +195,6 @@ class TestCaseAPI(CSRFExemptAPIView, TestCaseZipProcessor):
 class ProblemBase(APIView):
     def common_checks(self, request):
         data = request.data
-        if data["rule_type"] == ProblemRuleType.OI:
-            total_score = 0
-            for item in data["test_case_score"]:
-                if item["score"] <= 0:
-                    return "Invalid score"
-                else:
-                    total_score += item["score"]
-            data["total_score"] = total_score
         data["languages"] = list(data["languages"])
 
         # SQL 题校验：.sql 测试点与 .in/.out 沙箱判题互斥，SQL 必须是唯一语言