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/common/enums.ts

@@ -60,3 +60,23 @@ export enum BuildStatus {
   SUCCESS = 'success',
   FAILED = 'failed',
 }
+
+export enum BillingCycle {
+  HOURLY = 'hourly',
+  MONTHLY = 'monthly',
+  YEARLY = 'yearly',
+}
+
+export enum PricingResourceType {
+  BASE_FEE = 'base_fee',           // Base fee per runtime type
+  CPU_PER_CORE = 'cpu_per_core',   // Price per CPU core
+  MEMORY_PER_GB = 'memory_per_gb', // Price per GB RAM
+  STORAGE_PER_GB = 'storage_per_gb', // Price per GB disk
+  DATABASE_ADDON = 'database_addon', // Price for database addon
+}
+
+export enum TransactionType {
+  CHARGE = 'charge',       // Top-up / deposit
+  DEDUCTION = 'deduction', // Payment for service
+  REFUND = 'refund',       // Refund
+}