feat(plugin): 级联删除 — relations OnDeleteStrategy 支持

delete 方法扩展为处理三种级联策略：Restrict（存在关联时拒绝删除）、
Nullify（置空外键字段）、Cascade（级联软删除关联记录）。
在软删除主记录之前按声明顺序处理所有关联关系。

crates/erp-plugin/src/data_service.rs

@@ -262,6 +262,65 @@ impl PluginDataService {
         _event_bus: &EventBus,
     ) -> AppResult<()> {
         let info = resolve_entity_info(plugin_id, entity_name, tenant_id, db).await?;
+
+        // 解析 entity schema 获取 relations
+        let entity_def: crate::manifest::PluginEntity =
+            serde_json::from_value(info.schema_json.clone())
+                .map_err(|e| AppError::Internal(format!("解析 entity schema 失败: {}", e)))?;
+
+        let manifest_id = resolve_manifest_id(plugin_id, tenant_id, db).await?;
+
+        // 处理级联关系
+        for relation in &entity_def.relations {
+            let rel_table = DynamicTableManager::table_name(&manifest_id, &relation.entity);
+            let fk = sanitize_identifier(&relation.foreign_key);
+
+            match relation.on_delete {
+                crate::manifest::OnDeleteStrategy::Restrict => {
+                    let check_sql = format!(
+                        "SELECT 1 as chk FROM \"{}\" WHERE data->>'{}' = $1 AND tenant_id = $2 AND deleted_at IS NULL LIMIT 1",
+                        rel_table, fk
+                    );
+                    #[derive(FromQueryResult)]
+                    struct RefCheck { chk: Option<i32> }
+                    let has_ref = RefCheck::find_by_statement(Statement::from_sql_and_values(
+                        sea_orm::DatabaseBackend::Postgres,
+                        check_sql,
+                        [id.to_string().into(), tenant_id.into()],
+                    )).one(db).await?;
+                    if has_ref.is_some() {
+                        return Err(AppError::Validation(format!(
+                            "存在关联的 {} 记录，无法删除",
+                            relation.entity
+                        )));
+                    }
+                }
+                crate::manifest::OnDeleteStrategy::Nullify => {
+                    let nullify_sql = format!(
+                        "UPDATE \"{}\" SET data = jsonb_set(data, '{{{}}}', 'null'), updated_at = NOW() WHERE data->>'{}' = $1 AND tenant_id = $2 AND deleted_at IS NULL",
+                        rel_table, fk, fk
+                    );
+                    db.execute(Statement::from_sql_and_values(
+                        sea_orm::DatabaseBackend::Postgres,
+                        nullify_sql,
+                        [id.to_string().into(), tenant_id.into()],
+                    )).await?;
+                }
+                crate::manifest::OnDeleteStrategy::Cascade => {
+                    let cascade_sql = format!(
+                        "UPDATE \"{}\" SET deleted_at = NOW(), updated_at = NOW() WHERE data->>'{}' = $1 AND tenant_id = $2 AND deleted_at IS NULL",
+                        rel_table, fk
+                    );
+                    db.execute(Statement::from_sql_and_values(
+                        sea_orm::DatabaseBackend::Postgres,
+                        cascade_sql,
+                        [id.to_string().into(), tenant_id.into()],
+                    )).await?;
+                }
+            }
+        }
+
+        // 软删除主记录
         let (sql, values) = DynamicTableManager::build_delete_sql(&info.table_name, id, tenant_id);
 
         db.execute(Statement::from_sql_and_values(