feat(ids): 实现基于Snowflake的分布式ID生成功能

新增rs-snowflake依赖并实现分布式ID生成工具
在utils模块中添加ids子模块，提供业务ID生成与解析功能
替换原有UUID生成方式为分布式ID生成器

backend/src/services/flow_service.rs

@@ -98,7 +98,7 @@ pub async fn create(db: &Db, req: FlowCreateReq) -> anyhow::Result<FlowDoc> {
         let _parsed: FlowDSL = serde_yaml::from_str(yaml).context("invalid flow yaml")?;
         info!(target: "udmin", "flow.create: yaml parsed ok");
     }
-    let id = uuid::Uuid::new_v4().to_string();
+    let id = crate::utils::generate_flow_id();
     let name = req
         .name
         .clone()