chore(pl): add pl_plan.create_date migration and DDL snapshot

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/scripts/migrate_20260727_pl_plan_create_date.sql

@@ -0,0 +1,16 @@
+-- backend/scripts/migrate_20260727_pl_plan_create_date.sql
+-- 运输计划：业务创建日期 create_date（与系统 create_time 独立）
+
+ALTER TABLE `pl_plan`
+  ADD COLUMN `create_date` date DEFAULT NULL COMMENT '创建日期' AFTER `plan_no`;
+
+UPDATE `pl_plan`
+SET `create_date` = DATE(`create_time`)
+WHERE `create_date` IS NULL AND `create_time` IS NOT NULL;
+
+UPDATE `pl_plan`
+SET `create_date` = CURDATE()
+WHERE `create_date` IS NULL;
+
+ALTER TABLE `pl_plan`
+  MODIFY COLUMN `create_date` date NOT NULL COMMENT '创建日期';