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/billing/billing.module.ts

@@ -0,0 +1,18 @@
+import { Module } from '@nestjs/common';
+import { TypeOrmModule } from '@nestjs/typeorm';
+import { BillingService } from './billing.service';
+import { BillingController } from './billing.controller';
+import { ServicePlan } from './entities/service-plan.entity';
+import { PricingRule } from './entities/pricing-rule.entity';
+import { Wallet } from './entities/wallet.entity';
+import { WalletTransaction } from './entities/wallet-transaction.entity';
+
+@Module({
+  imports: [
+    TypeOrmModule.forFeature([ServicePlan, PricingRule, Wallet, WalletTransaction]),
+  ],
+  controllers: [BillingController],
+  providers: [BillingService],
+  exports: [BillingService],
+})
+export class BillingModule {}