Grades 4 and 5, Bazaar purchase from the app, pack SKUs

- rag: MATH_4 / MATH_5 textbooks (TOC and page offsets read from the PDFs)
- content: grades 4 and 5 in the seed files: 7 units / 28 nodes each,
  204 + 194 questions (every node >= 4), all re-solved blind once
- subscription: Plan.bazaar_sku (admin); POST /api/subscription/purchase
  accepts paymentMethod=bazaar for packs, rejects unknown methods (was ignored)
- payment: idempotency key cannot switch provider; discount codes refused
  on Bazaar orders (Bazaar charges the fixed SKU price)

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

subscription/migrations/0012_plan_bazaar_sku.py

@@ -0,0 +1,22 @@
+# Generated by Django 5.2.17 on 2026-10-03 12:55
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('subscription', '0011_questioncredit_refund_reason'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='plan',
+            name='bazaar_sku',
+            field=models.CharField(blank=True, default='', help_text='SKUِ محصولِ درون\u200cبرنامه\u200cای در پیشخانِ کافه\u200cبازار؛ خالی = در بازار فروخته نمی\u200cشود.', max_length=100),
+        ),
+        migrations.AddConstraint(
+            model_name='plan',
+            constraint=models.UniqueConstraint(condition=models.Q(('bazaar_sku', ''), _negated=True), fields=('bazaar_sku',), name='plan_bazaar_sku_unique_when_set'),
+        ),
+    ]