fix(c2c-bot): 调整purchaseOrder操作顺序，先扣余额再链上转账

问题：原流程先执行不可逆的链上dUSDT转账，再扣减卖家DB余额。
若转账成功但扣减失败（余额不足、DB异常），会导致：
- dUSDT已转出（链上不可回退）
- 订单仍为PENDING（可能被重复处理）
- 卖家余额未扣减

修复：
1. 将deductSellerBalance移至transferDusdt之前（可逆操作先行）
2. 链上转账失败时调用restoreSellerBalance回补余额
3. 新增restoreSellerBalance方法，失败时记录CRITICAL级别日志

修改后流程：扣余额(可逆) → 链上转账(不可逆) → 更新订单状态(DB)
任何步骤失败都不会导致资金损失。

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

backend/services/trading-service/src/application/services/c2c-bot.service.ts

@@ -47,15 +47,24 @@ export class C2cBotService {
       }
       this.logger.log(`[BOT] Seller Kava address: ${kavaAddress}`);
 
-      // 2. 计算 dUSDT 支付金额（积分值 = dUSDT，1:1 兑换）
+      // 2. 先扣减卖家积分值余额（可逆的DB操作，放在不可逆的链上转账之前）
       const paymentAmount = order.totalAmount;
-      this.logger.log(`[BOT] Payment amount: ${paymentAmount} dUSDT`);
+      await this.deductSellerBalance(order.makerAccountSequence, paymentAmount);
+      this.logger.log(`[BOT] Deducted seller balance, proceeding with on-chain transfer`);
 
-      // 3. 调用 mining-blockchain-service 转账 dUSDT
+      // 3. 调用 mining-blockchain-service 转账 dUSDT（不可逆）
-      const transferResult = await this.blockchainClient.transferDusdt(kavaAddress, paymentAmount);
+      let transferResult;
+      try {
+        transferResult = await this.blockchainClient.transferDusdt(kavaAddress, paymentAmount);
+      } catch (error: any) {
+        this.logger.error(`[BOT] Transfer exception: ${error.message}, restoring seller balance`);
+        await this.restoreSellerBalance(order.makerAccountSequence, paymentAmount);
+        return false;
+      }
 
       if (!transferResult.success) {
-        this.logger.error(`[BOT] Transfer failed: ${transferResult.error}`);
+        this.logger.error(`[BOT] Transfer failed: ${transferResult.error}, restoring seller balance`);
+        await this.restoreSellerBalance(order.makerAccountSequence, paymentAmount);
         return false;
       }
 
@@ -67,9 +76,6 @@ export class C2cBotService {
         paymentTxHash: transferResult.txHash!,
       });
 
-      // 5. 扣减卖家的积分值余额
-      await this.deductSellerBalance(order.makerAccountSequence, order.totalAmount);
-
       this.logger.log(`[BOT] Order ${order.orderNo} completed successfully`);
       return true;
     } catch (error: any) {
@@ -96,7 +102,6 @@ export class C2cBotService {
       throw new Error(`Insufficient cash balance for ${accountSequence}`);
     }
 
-    // 扣减余额
     await this.tradingAccountRepository.updateCashBalance(
       accountSequence,
       amountDecimal.negated().toString(),
@@ -105,6 +110,22 @@ export class C2cBotService {
     this.logger.log(`[BOT] Deducted ${amount} from ${accountSequence}'s cash balance`);
   }
 
+  /**
+   * 回补卖家的积分值余额（链上转账失败时的补偿操作）
+   */
+  private async restoreSellerBalance(accountSequence: string, amount: string): Promise<void> {
+    try {
+      await this.tradingAccountRepository.updateCashBalance(
+        accountSequence,
+        amount, // 正数，加回余额
+      );
+      this.logger.log(`[BOT] Restored ${amount} to ${accountSequence}'s cash balance`);
+    } catch (error: any) {
+      // 回补失败是严重问题，必须告警
+      this.logger.error(`[BOT] CRITICAL: Failed to restore ${amount} to ${accountSequence}: ${error.message}`);
+    }
+  }
+
   /**
    * 检查 Bot 服务是否可用
    */