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/entities/wallet-transaction.entity.ts

@@ -0,0 +1,41 @@
+import {
+  Entity,
+  PrimaryGeneratedColumn,
+  Column,
+  ManyToOne,
+  JoinColumn,
+  CreateDateColumn,
+} from 'typeorm';
+import { TransactionType } from '../../common/enums';
+import { Wallet } from './wallet.entity';
+
+@Entity('wallet_transactions')
+export class WalletTransaction {
+  @PrimaryGeneratedColumn('uuid')
+  id: string;
+
+  @Column({ type: 'enum', enum: TransactionType })
+  type: TransactionType;
+
+  @Column({ type: 'decimal', precision: 14, scale: 2 })
+  amount: number; // Amount in Toman (positive)
+
+  @Column({ type: 'decimal', precision: 14, scale: 2 })
+  balanceAfter: number; // Balance after this transaction
+
+  @Column({ nullable: true })
+  description: string; // e.g. "Charge via admin", "Payment for app: my-app (monthly)"
+
+  @Column({ nullable: true })
+  applicationId: string; // Linked application (for deductions)
+
+  @ManyToOne(() => Wallet, (wallet: Wallet) => wallet.transactions, { onDelete: 'CASCADE' })
+  @JoinColumn({ name: 'walletId' })
+  wallet: Wallet;
+
+  @Column()
+  walletId: string;
+
+  @CreateDateColumn()
+  createdAt: Date;
+}