feat: add is_recommended to Course and allow_multiple_attempts to Quiz

- Add is_recommended field to Course model (default: false)
- Add allow_multiple_attempts field to Quiz model (default: true)
- Create migration: add_recommended_and_quiz_attempts

Backend/prisma/schema.prisma

@@ -137,6 +137,7 @@ model Course {
   price            Decimal      @default(0) @db.Decimal(10, 2)
   is_free          Boolean      @default(false)
   have_certificate Boolean      @default(false)
+  is_recommended   Boolean      @default(false)
   status           CourseStatus @default(DRAFT)
   approved_by      Int?
   approved_at      DateTime?
@@ -304,6 +305,7 @@ model Quiz {
   shuffle_choices               Boolean   @default(false)
   show_answers_after_completion Boolean   @default(true)
   is_skippable                  Boolean   @default(true)
+  allow_multiple_attempts       Boolean   @default(true)
   created_at                    DateTime  @default(now())
   created_by                    Int
   updated_at                    DateTime? @updatedAt