fix(migration): 使迁移脚本幂等以支持重试

迁移脚本添加 IF NOT EXISTS 检查，避免重复执行时失败

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

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

backend/services/planting-service/prisma/migrations/20241225100000_add_contract_no/migration.sql

@@ -1,16 +1,44 @@
 -- AlterTable: Add contract_no column to contract_signing_tasks
 -- 合同编号格式: accountSequence-yyyyMMddHHmm (例如: 10001-202512251003)
 
--- 1. 首先添加可空的 contract_no 列
-ALTER TABLE "contract_signing_tasks" ADD COLUMN "contract_no" VARCHAR(30);
+-- 1. 首先添加可空的 contract_no 列（如果不存在）
+DO $$
+BEGIN
+    IF NOT EXISTS (
+        SELECT 1 FROM information_schema.columns
+        WHERE table_name = 'contract_signing_tasks'
+        AND column_name = 'contract_no'
+    ) THEN
+        ALTER TABLE "contract_signing_tasks" ADD COLUMN "contract_no" VARCHAR(30);
+    END IF;
+END $$;
 
 -- 2. 为现有记录生成合同编号（基于 account_sequence 和 created_at）
 UPDATE "contract_signing_tasks"
 SET "contract_no" = "account_sequence" || '-' || TO_CHAR("created_at", 'YYYYMMDDHH24MI')
 WHERE "contract_no" IS NULL;
 
--- 3. 设置为 NOT NULL
-ALTER TABLE "contract_signing_tasks" ALTER COLUMN "contract_no" SET NOT NULL;
+-- 3. 设置为 NOT NULL（如果还不是）
+DO $$
+BEGIN
+    IF EXISTS (
+        SELECT 1 FROM information_schema.columns
+        WHERE table_name = 'contract_signing_tasks'
+        AND column_name = 'contract_no'
+        AND is_nullable = 'YES'
+    ) THEN
+        ALTER TABLE "contract_signing_tasks" ALTER COLUMN "contract_no" SET NOT NULL;
+    END IF;
+END $$;
 
--- 4. 添加唯一索引
-CREATE UNIQUE INDEX "contract_signing_tasks_contract_no_key" ON "contract_signing_tasks"("contract_no");
+-- 4. 添加唯一索引（如果不存在）
+DO $$
+BEGIN
+    IF NOT EXISTS (
+        SELECT 1 FROM pg_indexes
+        WHERE tablename = 'contract_signing_tasks'
+        AND indexname = 'contract_signing_tasks_contract_no_key'
+    ) THEN
+        CREATE UNIQUE INDEX "contract_signing_tasks_contract_no_key" ON "contract_signing_tasks"("contract_no");
+    END IF;
+END $$;