feat(bx): add occur_date column on reimburse detail

Store optional expense occurrence date (DATE, nullable) for each
reimburse line item.

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

backend/scripts/migrate_20260817_bx_reimburse_detail_occur_date.sql

@@ -0,0 +1,6 @@
+-- backend/scripts/migrate_20260817_bx_reimburse_detail_occur_date.sql
+-- 报销明细增加可选发生日期 occur_date
+
+ALTER TABLE `bx_reimburse_detail`
+  ADD COLUMN `occur_date` date DEFAULT NULL COMMENT '费用发生日期'
+  AFTER `amount`;