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>

config/settings.py

@@ -410,7 +410,10 @@ REST_FRAMEWORK = {
         'token_refresh': '30/hour',
         'question_answer': '60/min',
         'question_report': '20/hour',
-        'lesson_complete': '10/hour',
+        # هر درس اینجا یک نوع مسئله است با یکی-دو سوال، نه یک جلسه‌ی درسی؛
+        # ۱۰ در ساعت را دانش‌آموزِ عادی می‌زد و پیشرفتش ۴۲۹ می‌گرفت.
+        'lesson_complete': '120/hour',
+        'node_complete': '60/hour',
         'support_create': '30/hour',
         'offer_redeem': '10/hour',
         'discount_validate': '20/hour',