Overview of the 13 backend apps, shared design patterns, serializers, viewsets, and models.

Backend Overview & Engineering Standards

IMPLEMENTED

The backend is built with Django 6.1 and Django REST Framework (DRF) 3.18. All applications reside under backend/apps/.

1. Application Inventory (13 Apps, 52 Models)

Application	Path	Model Count	Key Models
`apps.core`	`backend/apps/core`	10	`Tenant`, `TenantDomain`, `AuditLog`, `SaaSPackage`, `DatabaseBackup`
`apps.authentication`	`backend/apps/authentication`	6	`StaffProfile`, `Role`, `Permission`, `StaffMembership`, `Reseller`
`apps.customers`	`backend/apps/customers`	1	`Customer`
`apps.billing`	`backend/apps/billing`	5	`Package`, `Invoice`, `Recharge`, `Offer`, `ResellerPricing`
`apps.finance`	`backend/apps/finance`	6	`BillingAccount`, `InvoiceLine`, `LedgerEntry`, `Adjustment`, `IdempotencyKey`
`apps.payments`	`backend/apps/payments`	5	`PaymentGateway`, `PaymentTransaction`, `InboundPaymentEvent`, `SmsLog`
`apps.network`	`backend/apps/network`	5	`POPBranch`, `Router`, `OLT`, `ONU`, `UserSession`
`apps.support`	`backend/apps/support`	2	`Ticket`, `TicketReply`
`apps.hr`	`backend/apps/hr`	5	`Employee`, `Attendance`, `LeaveRequest`, `AdvanceSalary`, `PayrollRecord`
`apps.store`	`backend/apps/store`	3	`ItemCategory`, `StoreItem`, `StockTransaction`
`apps.tasks`	`backend/apps/tasks`	1	`Task`
`apps.callcenter`	`backend/apps/callcenter`	3	`CallLog`, `VoiceSetting`, `VoiceTemplate`
`apps.reports`	`backend/apps/reports`	0	Analytical endpoints (`DashboardAnalyticsView`)

All API endpoints follow a consistent structure:

Inherit from viewsets.ModelViewSet or rest_framework.views.APIView.
Apply rest_framework.authentication.TokenAuthentication.
Enforce apps.core.permissions.IsTenantMember.
Restrict get_queryset() strictly to request.tenant.
Wrap financial and hardware mutations in transaction.atomic() and distributed_lock().