feat(pl): add pl_plan.copy_seq_no for copy-to-group sequence

Persist a tenant-scoped fixed serial for driver-plan clipboard text;
include migration backfill by create_time.

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

docs/sql/migrate_pl_plan_copy_seq_no_20261009.sql

@@ -0,0 +1,34 @@
+-- 运输计划复制发群固定序号（租户内流水）
+-- MySQL 5.7
+-- 2026-10-09
+-- 若列已存在，跳过 ALTER（重复执行会报 Duplicate column）。
+
+ALTER TABLE `pl_plan`
+  ADD COLUMN `copy_seq_no` int(11) DEFAULT NULL COMMENT '复制发群序号（租户内固定流水，首次复制时分配）' AFTER `plan_no`,
+  ADD KEY `idx_pl_plan_tenant_copy_seq` (`tenant_no`, `copy_seq_no`);
+
+-- 历史回填：按租户 create_time ASC, id ASC，含软删；已有值不覆盖
+-- MySQL 5.7 用户变量按租户编号（会话内执行）
+SET @prev_tenant := NULL;
+SET @rn := 0;
+
+UPDATE `pl_plan` p
+INNER JOIN (
+  SELECT
+    t.id,
+    t.seq_no
+  FROM (
+    SELECT
+      id,
+      tenant_no,
+      @rn := IF(@prev_tenant = tenant_no, @rn + 1, 1) AS seq_no,
+      @prev_tenant := tenant_no AS _tenant
+    FROM (
+      SELECT id, tenant_no, create_time
+      FROM `pl_plan`
+      ORDER BY tenant_no ASC, create_time ASC, id ASC
+    ) ordered
+  ) t
+) ranked ON ranked.id = p.id
+SET p.`copy_seq_no` = ranked.seq_no
+WHERE p.`copy_seq_no` IS NULL;