make every grade's id its class number and sort grades by it everywhere

Grade ids were the order the books were imported in (ninth=1, eighth=5), so
gradeId: 5 meant eighth grade and the admin, the question filter and the API
all listed grades out of order. Grade.number (1-12) is now read from the title,
new grades take it as their id, and migration 0008 moves existing ids plus every
reference to them: Unit, the loose grade_id in accounts/ai_tutor/rag, and the
grade target inside campaign JSON. Unnumbered demo grades move above 100 and the
id sequence is raised past 12 so they never collide with a real grade.

The frontend needs no change: it always reads gradeId from the server.

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

README.md

@@ -13,7 +13,7 @@
 | **آدرسِ زنده** | `https://api.math.tadrisino2.ir` · [سلامت](https://api.math.tadrisino2.ir/api/health) |
 | **استک** | Python 3.11+ · Django 5.2 · DRF · SQLite · Qdrant (RAG) |
 | **API** | ۵۶ اندپوینت · مستنداتِ زنده در `/api/docs/` |
-| **تست** | ۴۸۳ تست، همه سبز (`python manage.py test`) |
+| **تست** | ۴۸۹ تست، همه سبز (`python manage.py test`) |
 | **محتوا** | ۸ پایه · ۵۸ فصل · ۲۶۳ نود · ۲٬۶۱۷ درس · ۲٬۴۶۳ سوال |
 
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