- Add UpdateMerchantPlanCommand in MerchantService (Admin can set plan 0-3)
- Add PUT /api/v1/admin/merchants/{id}/plan endpoint
- Add BFF proxy PUT /api/bff/superadmin/merchants/{id}/plan with audit logging
- Add SubscriptionPlanId + SubscriptionPlanName to AdminMerchantListItemDto and AdminMerchantDetailDto
- Rewrite GetMerchantDetailQueryHandler to use EF joins (fix NullRef on Ignored nav properties)
- Add plan selector UI in MerchantDetail subscription tab (4 clickable plan cards)
- Save button appears when plan differs from current, with success/error feedback
- Add UpdateMerchantPlanAsync to SuperAdminApiService frontend
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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