feat: add course AI agent backend

prisma/schema.prisma

@@ -43,6 +43,21 @@ model User {
   @@map("users")
 }
 
+model CourseAiSetting {
+  id              String   @id @default(uuid()) @db.Uuid
+  courseId        String   @unique @map("course_id") @db.Uuid
+  enabled         Boolean  @default(false)
+  provider        String   @default("OPENAI")
+  model           String
+  encryptedApiKey String?  @map("encrypted_api_key")
+  createdAt       DateTime @default(now()) @map("created_at") @db.Timestamptz(3)
+  updatedAt       DateTime @updatedAt @map("updated_at") @db.Timestamptz(3)
+  course          Course   @relation(fields: [courseId], references: [id], onDelete: Cascade)
+
+  @@index([enabled])
+  @@map("course_ai_settings")
+}
+
 model StudentEnrollment {
   id               String    @id @default(uuid()) @db.Uuid
   userId           String    @map("user_id") @db.Uuid
@@ -173,6 +188,7 @@ model Course {
   submissionScores SubmissionScore[]
   studentGroups    StudentGroup[]
   submissionTotalScores SubmissionTotalScore[]
+  aiSetting CourseAiSetting?
 
   @@index([isActive])
   @@index([teacherId])