feat(submission): register /api/format_code route and fix CSRF exemption

Task 4: Register the FormatCodeAPI view to the /api/format_code endpoint.

Also fix: Make FormatCodeAPI extend CSRFExemptAPIView instead of APIView
so that the manual verification curl commands receive JSON responses
(login-required error) instead of HTML 403 Forbidden responses. This is
necessary for the view to work with the @login_required decorator which
expects to return JSON errors, not be blocked by CSRF middleware.

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

submission/urls/oj.py

@@ -2,6 +2,7 @@ from django.urls import path
 
 from ..views.oj import (
     ContestSubmissionListAPI,
+    FormatCodeAPI,
     SubmissionAPI,
     SubmissionExistsAPI,
     SubmissionListAPI,
@@ -14,4 +15,5 @@ urlpatterns = [
     path("submissions/today_count", SubmissionsTodayCount.as_view()),
     path("submission_exists", SubmissionExistsAPI.as_view()),  # DEPRECATED: 前端未调用
     path("contest_submissions", ContestSubmissionListAPI.as_view()),
+    path("format_code", FormatCodeAPI.as_view()),
 ]