fix(wallet-service): 将最小划转金额从100改为5

与前端配置保持一致

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/services/wallet-service/src/application/services/wallet-application.service.ts

@@ -1458,7 +1458,7 @@ export class WalletApplicationService {
   /**
    * 最小提现金额
    */
-  private readonly MIN_WITHDRAWAL_AMOUNT = 100; // 100 USDT
+  private readonly MIN_WITHDRAWAL_AMOUNT = 5; // 5 USDT
 
   /**
    * 请求提现