fix(aihr): persist org hire dates

backend/script/sql/update/aihr_20260714_org_hire_date_mysql8.sql

@@ -0,0 +1,17 @@
+-- AIHR 组织快照入职日期迁移（MySQL 8.x）
+-- 仅补齐结构，不写入业务数据；可重复执行。
+SET @aihr_hire_date_exists := (
+  SELECT COUNT(*)
+  FROM information_schema.COLUMNS
+  WHERE TABLE_SCHEMA = DATABASE()
+    AND TABLE_NAME = 'aihr_org_snapshot'
+    AND COLUMN_NAME = 'hire_date'
+);
+SET @aihr_hire_date_ddl := IF(
+  @aihr_hire_date_exists = 0,
+  'ALTER TABLE aihr_org_snapshot ADD COLUMN `hire_date` date DEFAULT NULL COMMENT ''入职日期（用于新员工三个月资格判断）'' AFTER `employment_status`',
+  'SELECT 1'
+);
+PREPARE aihr_hire_date_stmt FROM @aihr_hire_date_ddl;
+EXECUTE aihr_hire_date_stmt;
+DEALLOCATE PREPARE aihr_hire_date_stmt;