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>

notifications/models.py

@@ -9,6 +9,7 @@ class Notification(models.Model):
     TYPE_CHOICES = [
         ('xp_earned', 'امتیاز گرفتی'),
         ('lesson_completed', 'درس تکمیل شد'),
+        ('node_completed', 'جزیره تمام شد'),
         ('order_paid', 'پرداخت موفق'),
         ('ticket_replied', 'پاسخ تیکت'),
         ('family_link_accepted', 'اتصال خانواده تأیید شد'),