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>

web-admin/src/components/oa-approval/OaProgressChain.vue

@@ -228,7 +228,7 @@
   function resolveSourceText(src: string) {
     const map: Record<string, string> = {
       users: '指定成员',
-      oa_roles: '工作流角色',
+      posts: '职位',
       dept_leader: '部门负责人',
       direct_supervisor: '直属主管',
       supervisor_fallback_dept_leader: '部门负责人（主管回退）',