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

frontend/src/types/index.ts

@@ -238,3 +238,51 @@ export interface TicketStats {
   avgResponseTimeMinutes: number;
   byDepartment: Record<string, { total: number; open: number; answered: number; closed: number }>;
 }
+
+// ─── Billing types ──────────────────────────────────
+
+export type BillingCycle = 'hourly' | 'monthly' | 'yearly';
+export type PricingResourceType = 'base_fee' | 'cpu_per_core' | 'memory_per_gb' | 'storage_per_gb' | 'database_addon';
+export type TransactionType = 'charge' | 'deduction' | 'refund';
+
+export interface PricingRule {
+  id: string;
+  resourceType: PricingResourceType;
+  unitPrice: number;
+  description?: string;
+  planId: string;
+  createdAt: string;
+}
+
+export interface ServicePlan {
+  id: string;
+  name: string;
+  description?: string;
+  billingCycle: BillingCycle;
+  isActive: boolean;
+  pricingRules: PricingRule[];
+  createdAt: string;
+  updatedAt: string;
+}
+
+export interface WalletBalance {
+  balance: number;
+}
+
+export interface WalletTransaction {
+  id: string;
+  type: TransactionType;
+  amount: number;
+  balanceAfter: number;
+  description?: string;
+  applicationId?: string;
+  walletId: string;
+  createdAt: string;
+}
+
+export interface CostBreakdown {
+  hourly: number;
+  monthly: number;
+  yearly: number;
+  breakdown: { label: string; hourly: number; monthly: number; yearly: number }[];
+}