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>

content/serializers.py

@@ -13,7 +13,7 @@ class GradeSerializer(serializers.ModelSerializer):
 
     class Meta:
         model = Grade
-        fields = ['id', 'title', 'icon', 'color', 'lessonCount']
+        fields = ['id', 'number', 'title', 'icon', 'color', 'lessonCount']
 
 
 class GradesListResponseSerializer(serializers.Serializer):