feat: 发起时指派——assignPolicy 配置/assign_json 快照/引擎收窄/preview+apply 契约

src/main/java/com/yunhai/flowcenter/entity/FlwInstance.java

@@ -51,6 +51,10 @@ public class FlwInstance extends BaseEntity {
     @Column(name = "form_data_json", columnDefinition = "LONGTEXT")
     private String formDataJson;
 
+    /** 发起时指派快照：{nodeId:[{userId,userName}]}；NULL=无指派（全量候选现状行为） */
+    @Column(name = "assign_json", columnDefinition = "LONGTEXT")
+    private String assignJson;
+
     /** CMS 事项快照（原生模式，M1 不启用恒 null） */
     @Column(name = "item_code", length = 64)
     private String itemCode;