feat: implement billing module for tenant subscription management and super-admin payment processing

backend/src/app.module.ts

@@ -42,6 +42,7 @@ import { StorageModule } from './common/storage/storage.module';
 import { DocumentsModule } from './modules/documents/documents.module';
 import { VehiclesModule } from './modules/vehicles/vehicles.module';
 import { GeminiModule } from './integrations/gemini/gemini.module';
+import { BillingModule } from './modules/billing/billing.module';
 
 @Module({
   imports: [
@@ -98,6 +99,7 @@ import { GeminiModule } from './integrations/gemini/gemini.module';
     TenantWalletModule, // دفترا المستأجر: إيراد وأمانات (docs/24)
     CreditModule,
     PaymentsModule,
+    BillingModule,
     TripsModule,
     DispatchModule,
     ChatModule,