fix(出题页): 改了挑战点保存，提示「保存好了」其实一条都没存上

出题页发请求时漏了 challenges 这个字段，后端又把「不带」当成「不动原来的」，
所以改动被静默丢掉。现在出题页带上它，接口也改成必须带（null 是清空），
以后再漏传会直接报错，不会再悄悄丢。

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/api/src/routes/admin/problem.ts

@@ -260,9 +260,7 @@ type ProblemInput = ReturnType<typeof createProblemRequestSchema.parse>
 async function validateProblem(data: ProblemInput, previousChallenges: ProblemChallenges | null = null): Promise<{ error: string } | Validated> {
   const checked = commonChecks(data)
   if ('error' in checked) return checked
-  // 不带挑战（旧版出题页）就原样留着，见契约 createProblemRequestSchema.challenges
-  const normalized =
-    data.challenges === undefined ? { challenges: previousChallenges } : normalizeChallenges(data.challenges, previousChallenges, data.languages)
+  const normalized = normalizeChallenges(data.challenges, previousChallenges, data.languages)
   if ('error' in normalized) return normalized
   const { challenges } = normalized
   if (!checked.sql) return { sql: false, sqlDisplay: null, challenges }