feat: AI-powered quiz generation with caching

- Add Quiz model to store generated questions in DB
- Integrate OpenRouter API to generate quiz from lesson content
- Cache quiz results to avoid regenerating on every request
- Extract text from TipTap JSON and send to AI
- Fallback to default questions if AI fails or no API key

Co-authored-by: Cursor <cursoragent@cursor.com>

packages/database/prisma/schema.prisma

@@ -241,6 +241,7 @@ model Lesson {
   // Relations
   chapter     Chapter  @relation(fields: [chapterId], references: [id], onDelete: Cascade)
   homework    Homework[]
+  quiz        Quiz?
   
   // Vector embedding for semantic search
   embedding   Unsupported("vector(1536)")?
@@ -249,6 +250,18 @@ model Lesson {
   @@map("lessons")
 }
 
+model Quiz {
+  id          String   @id @default(uuid())
+  lessonId    String   @unique @map("lesson_id")
+  questions   Json     // Array of {id, question, options, correctAnswer}
+  
+  createdAt   DateTime @default(now()) @map("created_at")
+  
+  lesson      Lesson   @relation(fields: [lessonId], references: [id], onDelete: Cascade)
+  
+  @@map("quizzes")
+}
+
 // ============================================
 // AI Course Generation
 // ============================================