feat: support course-specific Qwen endpoints

prisma/migrations/20261014170000_add_course_ai_endpoint/migration.sql

@@ -0,0 +1,2 @@
+ALTER TABLE "course_ai_settings"
+ADD COLUMN "endpoint_url" TEXT;

prisma/schema.prisma

@@ -49,6 +49,7 @@ model CourseAiSetting {
   enabled         Boolean  @default(false)
   provider        String   @default("OPENAI")
   model           String
+  endpointUrl     String?  @map("endpoint_url")
   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)