fix(planting): 使用upsert解决合同签约任务创建的并发冲突

Kafka事件重复消费时,多个消费者同时创建签约任务会导致唯一约束冲突
改用upsert确保幂等性,如果orderNo已存在则返回现有记录

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

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

backend/services/planting-service/src/infrastructure/persistence/repositories/contract-signing-task.repository.impl.ts

@@ -41,9 +41,11 @@ export class ContractSigningTaskRepositoryImpl implements IContractSigningTaskRe
       });
       return this.mapToDomain(updated);
     } else {
-      // 创建
-      const created = await this.prisma.contractSigningTask.create({
-        data: {
+      // 创建 - 使用 upsert 处理并发幂等性
+      // 如果 orderNo 已存在，则只返回现有记录（不更新）
+      const result = await this.prisma.contractSigningTask.upsert({
+        where: { orderNo: task.orderNo },
+        create: {
           orderNo: task.orderNo,
           userId: task.userId,
           accountSequence: task.accountSequence,
@@ -62,8 +64,10 @@ export class ContractSigningTaskRepositoryImpl implements IContractSigningTaskRe
           status: task.status,
           expiresAt: task.expiresAt,
         },
+        // 如果已存在，不更新任何字段，只返回现有记录
+        update: {},
       });
-      return this.mapToDomain(created);
+      return this.mapToDomain(result);
     }
   }