feat: add is_skippable field to Quiz model with default value true

Add is_skippable boolean field to Quiz schema, update quiz creation and update logic to handle the new field, and include it in student course content responses. Update seed data and type definitions accordingly.

Backend/prisma/seed.js

@@ -299,6 +299,7 @@ async function main() {
         time_limit: 10,
         shuffle_questions: true,
         shuffle_choices: true,
+        is_skippable: true,
         created_by: instructor.id,
         questions: {
           create: [