feat: billing system — service plans, wallet, cost calculation

- Backend: billing module with ServicePlan, PricingRule, Wallet, WalletTransaction entities
- Admin can CRUD service plans with hourly/monthly/yearly billing cycles
- Each plan has flexible pricing rules (base_fee, cpu, memory, storage, db addon)
- Wallet system: auto-created per user, charge, deduct, refund, transaction history
- Cost calculation endpoint: cross-cycle conversion (hourly*720=monthly, monthly*12=yearly)
- Frontend: admin billing management page (/dashboard/admin/billing)
- Frontend: user wallet page with balance, quick-charge, transaction history
- Deploy page: cost breakdown shown in Review step (hourly/monthly/yearly)
- Navigation: Wallet link for users, Billing Plans link for admins

backend/src/app.module.ts

@@ -10,6 +10,7 @@ import { ClustersModule } from './clusters/clusters.module';
 import { KubernetesModule } from './kubernetes/kubernetes.module';
 import { BuildModule } from './build/build.module';
 import { TicketsModule } from './tickets/tickets.module';
+import { BillingModule } from './billing/billing.module';
 import configuration from './config/configuration';
 
 @Module({
@@ -58,6 +59,7 @@ import configuration from './config/configuration';
     KubernetesModule,
     BuildModule,
     TicketsModule,
+    BillingModule,
   ],
 })
 export class AppModule {}