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/admin.py

@@ -5,7 +5,8 @@ from .models import Plan, Subscription
 
 @admin.register(Plan)
 class PlanAdmin(admin.ModelAdmin):
-    list_display = ('id', 'title', 'price', 'currency', 'duration_days', 'ai_daily_quota', 'discount_percent', 'popular')
+    list_display = ('id', 'title', 'price', 'currency', 'duration_days', 'ai_daily_quota', 'discount_percent', 'popular',
+                    'bazaar_sku')
     list_editable = ('ai_daily_quota',)