fix: resolve circular import errors in TypeORM entities

- Use string-based @OneToMany relations in parent entities (wallet, ticket, service-plan)
  to break circular import chains that confused VS Code TS language server
- Add .vscode/settings.json to suppress Tailwind CSS @tailwindcss unknownAtRules warnings
- Reorder imports in child entities for consistency

backend/src/billing/entities/wallet-transaction.entity.ts

@@ -6,8 +6,8 @@ import {
   JoinColumn,
   CreateDateColumn,
 } from 'typeorm';
-import { TransactionType } from '../../common/enums';
 import { Wallet } from './wallet.entity';
+import { TransactionType } from '../../common/enums';
 
 @Entity('wallet_transactions')
 export class WalletTransaction {