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/migrations/0002_alter_notification_type.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.2.17 on 2026-09-23 23:10
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('notifications', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='notification',
+            name='type',
+            field=models.CharField(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'), ('weekly_report_ready', 'گزارش هفتگی آماده شد')], max_length=30),
+        ),
+    ]