feat: add Exercise model and CRUD API for tutorial exercises

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tutorial/urls/tutorial.py

@@ -1,7 +1,8 @@
 from django.urls import path
-from ..views.oj import TutorialAPI, TutorialTitlesAPI
+from ..views.oj import TutorialAPI, TutorialTitlesAPI, ExerciseAPI
 
 urlpatterns = [
     path("tutorial", TutorialAPI.as_view()),
     path("tutorials", TutorialTitlesAPI.as_view()),
+    path("exercises", ExerciseAPI.as_view()),
 ]