Subjects per grade (math, calculus, geometry, stats, discrete), 5 new textbooks, 202 new islands, long islands split, hand-written figures and lesson texts for the new books

- content.Subject + Unit.subject, /api/home?subject=, tutor grounded in the chosen book
- 5 new books ingested; ~1150 hand-written questions, islands named by hand
- node_split: islands over 7 lessons split into parts of <=6 (lesson ids and progress kept)
- Lesson.figures hand-authored and visually checked for 653 lessons of the new books;
  gen_lesson_figures got --subject and survives network errors
- seed files re-exported

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

content/migrations/0016_unit_subject.py

@@ -0,0 +1,20 @@
+import django.db.models.deletion
+from django.db import migrations, models
+
+import content.models
+
+
+class Migration(migrations.Migration):
+    dependencies = [('content', '0015_seed_subjects')]
+
+    operations = [
+        migrations.AddField(
+            model_name='unit',
+            name='subject',
+            field=models.ForeignKey(
+                default=content.models.default_subject_pk,
+                on_delete=django.db.models.deletion.PROTECT,
+                related_name='units', to='content.subject',
+            ),
+        ),
+    ]