fix(比赛): 比赛进行中，学生打开比赛题必 500

contest.ts 两处把「藏起来的难度」表示成空串，而 problemDifficultySchema
是严格的三值枚举，parse 直接抛 ZodError。contestDetailsAllowed 在
「比赛进行中 + 看的人不是管理员」时就是假 —— 也就是正常学生在正常比赛里，
比赛题列表和详情页一开就挂。dev 库 contest 表一直是空的，这条路径没被跑到过。

改成下发 null，契约里给这两个字段单独起了 maskedProblemDifficultySchema。
语义上对齐旧 Django：ProblemSafeSerializer 把 difficulty 放在 exclude 里，
字段整个不下发，而不是给一个假值。

端上顺着 null 补了四处：ProblemInfo 的「难度」项整条不渲染（和旧栈表现一致），
题目列表、题单列表同理，transforms 的 ProblemFiltered.difficulty 也放开为可空。
这几处是 vue-tsc 逐个揪出来的，正是严格枚举该起的作用。

实测（学生 / 超管 × 比赛题 / 普通题 四种组合）：学生看比赛题详情与列表
从 500 变 200 且 difficulty 为 null，超管仍拿到真值 Low，普通题不受影响；
页面上「难度」整项消失，统计面板其余内容正常。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016DHxhKNxXfG89JnVzHbvgj

apps/api/src/routes/contest.ts

@@ -144,7 +144,7 @@ contestRoutes.get("/contests/:id/problems", optionalAuth, requireContestAccess("
     title: problem.title,
     submissionNumber: allowed ? problem.submissionNumber : 0,
     acceptedNumber: allowed ? problem.acceptedNumber : 0,
-    difficulty: allowed ? problem.difficulty : "",
+    difficulty: allowed ? problem.difficulty : null,
     createdBy: sampleUser(user, realName),
     tags: tags.get(problem.id) ?? [],
     contestId: contest.id,
@@ -179,7 +179,7 @@ contestRoutes.get("/contests/:id/problems/:displayId", optionalAuth, requireCont
     lastUpdateTime: row.problem.lastUpdateTime,
     timeLimit: row.problem.timeLimit,
     memoryLimit: row.problem.memoryLimit,
-    difficulty: allowed ? row.problem.difficulty : "",
+    difficulty: allowed ? row.problem.difficulty : null,
     source: row.problem.source,
     prompt: row.problem.prompt,
     submissionNumber: allowed ? row.problem.submissionNumber : 0,

apps/web/src/oj/problem/components/ProblemInfo.vue

@@ -121,7 +121,8 @@ onMounted(() => {
     <n-descriptions-item label="创建时间">
       {{ parseTime(problem.createTime) }}
     </n-descriptions-item>
-    <n-descriptions-item label="难度">
+    <!-- 比赛进行中，难度不下发（difficulty 为 null），这一项整个不显示 -->
+    <n-descriptions-item v-if="problem.difficulty" label="难度">
       <n-tag :type="getTagColor(problem.difficulty)">
         {{ DIFFICULTY[problem.difficulty] }}
       </n-tag>

apps/web/src/oj/problem/list.vue

@@ -173,7 +173,9 @@ const baseColumns: DataTableColumn<ProblemFiltered>[] = [
     key: "difficulty",
     width: 100,
     render: (row) =>
-      h(NTag, { type: getTagColor(row.difficulty) }, () => row.difficulty),
+      row.difficulty
+        ? h(NTag, { type: getTagColor(row.difficulty) }, () => row.difficulty)
+        : null,
   },
   {
     title: renderTableTitle("标签", "streamline-ultimate-color:attachment"),

apps/web/src/oj/problemset/components/ProblemSetProblemsList.vue

@@ -54,6 +54,7 @@ function handleProblemClick(problemId: string) {
 
               <n-flex align="center" size="small">
                 <n-tag
+                  v-if="problemSetProblem.problem.difficulty"
                   :type="getTagColor(problemSetProblem.problem.difficulty)"
                   size="small"
                 >

apps/web/src/oj/transforms.ts

@@ -8,7 +8,7 @@ export function filterResult(result: ProblemListItem): ProblemFiltered {
     id: result.id,
     _id: result._id,
     title: result.title,
-    difficulty: DIFFICULTY[result.difficulty],
+    difficulty: result.difficulty ? DIFFICULTY[result.difficulty] : null,
     tags: result.tags,
     submission: result.submissionNumber,
     rate: getACRate(result.acceptedNumber, result.submissionNumber),

apps/web/src/utils/types.ts

@@ -176,7 +176,8 @@ export interface ProblemFiltered {
   _id: string
   id: number
   title: string
-  difficulty: "简单" | "中等" | "困难"
+  // 比赛进行中难度不下发，见 contract 的 maskedProblemDifficultySchema
+  difficulty: "简单" | "中等" | "困难" | null
   tags: string[]
   submission: number
   rate: string