feat(oa): add oa_instance.initiator_select_json for per-node self-select

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

docs/sql/patch_2026-07-23_oa_instance_initiator_select_json.sql

@@ -0,0 +1,4 @@
+-- OA 实例：按节点持久化发起人自选（nodeId -> userId[]）
+ALTER TABLE oa_instance
+  ADD COLUMN initiator_select_json TEXT NULL COMMENT '发起人自选 JSON {nodeId:[userId,...]}'
+  AFTER form_data_json;