feat(db): add production migration chain

deploy/sql/migrations/overseas/baiyue_lastleg/V5__allow_multiple_relabel_applications.sql

@@ -0,0 +1,14 @@
+-- Region: OVERSEAS
+-- Schema: baiyue_lastleg
+-- Purpose: 允许同一尾程订单建立多个换标申请，并保留订单查询索引。
+-- Preconditions: 连接必须指向 baiyue_lastleg；执行前确认旧唯一索引不再是业务约束。
+-- Postconditions: ll_relabel_application.lastleg_order_id 不再受唯一索引限制，普通查询索引存在。
+
+SET NAMES utf8mb4;
+
+SET @ddl = IF(EXISTS (SELECT 1 FROM information_schema.statistics WHERE table_schema = DATABASE() AND table_name = 'll_relabel_application' AND index_name = 'uk_ll_relabel_application_order'),
+  'ALTER TABLE ll_relabel_application DROP INDEX uk_ll_relabel_application_order', 'SELECT 1');
+PREPARE stmt FROM @ddl; EXECUTE stmt; DEALLOCATE PREPARE stmt;
+SET @ddl = IF(NOT EXISTS (SELECT 1 FROM information_schema.statistics WHERE table_schema = DATABASE() AND table_name = 'll_relabel_application' AND index_name = 'idx_ll_relabel_application_order'),
+  'ALTER TABLE ll_relabel_application ADD KEY idx_ll_relabel_application_order (lastleg_order_id)', 'SELECT 1');
+PREPARE stmt FROM @ddl; EXECUTE stmt; DEALLOCATE PREPARE stmt;