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/serializers.py

@@ -136,6 +136,23 @@ class NodeQuizResponseSerializer(serializers.Serializer):
     questions = QuizQuestionSerializer(many=True)
 
 
+class NodeQuizCompleteRequestSerializer(serializers.Serializer):
+    # همان شکلِ lessons/{id}/complete تا فرانت یک الگو یاد بگیرد. سقفِ
+    # correctCount در view با تعدادِ واقعیِ سوال‌های مبحث چک می‌شود.
+    correctCount = serializers.IntegerField(min_value=0)
+    totalQuestions = serializers.IntegerField(min_value=1)
+
+
+class NodeQuizCompleteResponseSerializer(serializers.Serializer):
+    nodeId = serializers.CharField()
+    stars = serializers.IntegerField()
+    xpEarned = serializers.IntegerField()
+    coinsEarned = serializers.IntegerField()
+    # جزیره‌ی بعدی که حالا باز است؛ null یعنی این آخرین جزیره‌ی پایه بود.
+    nextNodeId = serializers.CharField(allow_null=True)
+    stats = StatsSerializer()
+
+
 class LessonQuestionsResponseSerializer(serializers.Serializer):
     questions = QuestionSerializer(many=True)