fix: User + KnowledgeBase 补充 Quiz 反向关联

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

prisma/schema.prisma

@@ -37,6 +37,7 @@ model User {
   learningSessions        LearningSession[]
   learningRecords         LearningRecord[]
   activeRecallQuestions   ActiveRecallQuestion[]
+  quizAttempts            QuizAttempt[]
   activeRecallAnswers     ActiveRecallAnswer[]
   aiAnalysisJobs          AiAnalysisJob[]
   aiAnalysisResults       AiAnalysisResult[]
@@ -189,6 +190,7 @@ model KnowledgeBase {
   focusItems    FocusItem[]
   folders       KnowledgeFolder[]
   subscriptions KnowledgeBaseSubscription[]
+  quizzes      Quiz[]
 
   @@index([userId])
   @@index([status])