fix(c2c): 用updateMany+status条件防止合并时的竞态条件

findFirst找到PENDING订单后，如果被takeOrder接走（状态变MATCHED），
updateMany因status!=PENDING返回count=0，自动回退到创建新订单。

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

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

@@ -785,7 +785,7 @@ export class C2cService {
           const priceDecimal = new Decimal(freshOrder.price);
 
           // 查找是否已有可合并的 PENDING 订单（同 maker + 同类型 + 同价格）
-          const existingPending = await tx.c2cOrder.findFirst({
+          let existingPending = await tx.c2cOrder.findFirst({
             where: {
               makerAccountSequence: freshOrder.makerAccountSequence,
               type: freshOrder.type as any,
@@ -799,21 +799,35 @@ export class C2cService {
             const existingQty = new Decimal(existingPending.quantity.toString());
             const mergedQty = existingQty.plus(quantityDecimal);
             const mergedTotal = priceDecimal.mul(mergedQty).toString();
-            restoreOrderNo = existingPending.orderNo;
 
             this.logger.log(
-              `[EXPIRY][TX] 步骤3: 合并到已有 PENDING 订单 ${existingPending.orderNo}, ` +
+              `[EXPIRY][TX] 步骤3: 尝试合并到 PENDING 订单 ${existingPending.orderNo}, ` +
               `原数量=${existingQty} + 恢复=${quantityDecimal} = ${mergedQty}`,
             );
 
-            await tx.c2cOrder.update({
-              where: { orderNo: existingPending.orderNo },
+            // 用 updateMany + status 条件防止竞态：如果订单在 findFirst 后被 takeOrder 接走，count=0
+            const updateResult = await tx.c2cOrder.updateMany({
+              where: {
+                orderNo: existingPending.orderNo,
+                status: C2C_ORDER_STATUS.PENDING as any,
+              },
               data: {
                 quantity: mergedQty.toString(),
                 totalAmount: mergedTotal,
               },
             });
-          } else {
+
+            if (updateResult.count > 0) {
+              restoreOrderNo = existingPending.orderNo;
+              this.logger.log(`[EXPIRY][TX] 步骤3: 合并成功`);
+            } else {
+              // 订单已被接走（状态不再是 PENDING），回退到创建新订单
+              this.logger.warn(`[EXPIRY][TX] 步骤3: 订单 ${existingPending.orderNo} 已被接走，改为创建新订单`);
+              existingPending = null; // fall through to create
+            }
+          }
+
+          if (shouldRestore && !restoreOrderNo) {
             // 无可合并订单，创建新的 PENDING 订单
             restoreOrderNo = this.generateOrderNo();
             const totalAmount = priceDecimal.mul(quantityDecimal).toString();