let a finished island quiz open the next island, and ground ask-AI in the student's book

The frontend plays node quizzes, but a node only completed once every one of its
8-13 lessons was completed separately, so students were stuck after the first
island of each unit. POST /api/nodes/{id}/quiz/complete now completes the node
(any score; score only sets stars/xp) and returns nextNodeId.

Also closes the other backend gaps found while auditing the frontend:
- /api/ai/ask without lessonId now picks the book from the user's grade instead
  of skipping retrieval, so the "ask Pi" tab is grounded in the textbook.
- guest sessions return a refreshToken; they used to go silently 401 after 1h.
- lesson_complete throttle 10/hour -> 120/hour (a lesson here is 1-2 questions).
- progressPercent is per grade, not over all 2,617 lessons of every grade.
- a one-year plan (299,000 toman) is created by migration when no plan exists.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

content/urls.py

@@ -7,6 +7,7 @@ from .views import (
     LessonIntroView,
     LessonQuestionsView,
     NodeLessonsView,
+    NodeQuizCompleteView,
     NodeQuizView,
     NodeProgressListView,
     QuestionAnswerView,
@@ -23,5 +24,6 @@ urlpatterns = [
     path('questions/<int:question_id>/report', QuestionReportView.as_view(), name='question-report'),
     path('nodes/<int:node_id>/lessons', NodeLessonsView.as_view(), name='node-lessons'),
     path('nodes/<int:node_id>/quiz', NodeQuizView.as_view(), name='node-quiz'),
+    path('nodes/<int:node_id>/quiz/complete', NodeQuizCompleteView.as_view(), name='node-quiz-complete'),
     path('progress/nodes', NodeProgressListView.as_view(), name='node-progress-list'),
 ]