feat(permission): replace OA roles with post-bound roles and assignee posts

Unify effective roles via post-role bindings, remove dedicated OA role UI, and update OA assignee resolution plus related SQL/docs.

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

docs/sql/drop_oa_role.sql

@@ -0,0 +1,10 @@
+-- docs/sql/drop_oa_role.sql
+-- 下线 OA 工作流角色：审批人选改为职位（sys_post / sys_user_post）
+-- 同步软删菜单「工作流角色」
+
+UPDATE `sys_menu`
+SET `del_flag` = 1, `update_time` = NOW()
+WHERE `id` = 'OA000000000000000000000000000102' AND `del_flag` = 0;
+
+DROP TABLE IF EXISTS `oa_role_user`;
+DROP TABLE IF EXISTS `oa_role`;