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/0014_subject.py

@@ -0,0 +1,23 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    """مرحله‌ی ۱ از ۴ (جدولِ کتاب‌ها). چهار مرحله‌ی جدا چون PostgreSQL اجازه نمی‌دهد در یک تراکنش
+    هم داده نوشت و هم همان جدول را ALTER کرد (pending trigger events)."""
+
+    dependencies = [('content', '0013_alter_userlessoncompletion_created_at_and_more')]
+
+    operations = [
+        migrations.CreateModel(
+            name='Subject',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('key', models.SlugField(max_length=30, unique=True)),
+                ('title', models.CharField(max_length=50, unique=True)),
+                ('icon', models.CharField(blank=True, max_length=100)),
+                ('color', models.CharField(blank=True, max_length=10)),
+                ('order', models.PositiveSmallIntegerField(default=0)),
+            ],
+            options={'ordering': ['order', 'id']},
+        ),
+    ]