feat(message): add cross-module event integration (Phase 6)

- Message module subscribes to workflow events (process_instance.started)
- Auto-generates notifications when workflows start
- Added started_by to workflow instance event payload
- Event listener runs as background tokio task

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/erp-message/src/module.rs

@@ -57,6 +57,43 @@ impl MessageModule {
                 put(subscription_handler::update_subscription),
             )
     }
+
+    /// 启动后台事件监听任务，将工作流事件转化为消息通知。
+    ///
+    /// 在 main.rs 中调用，因为需要 db 连接。
+    pub fn start_event_listener(db: sea_orm::DatabaseConnection, event_bus: EventBus) {
+        use sea_orm::ConnectionTrait;
+
+        let mut rx = event_bus.subscribe();
+        tokio::spawn(async move {
+            loop {
+                match rx.recv().await {
+                    Ok(event) => {
+                        let db = db.clone();
+                        let event_bus = event_bus.clone();
+                        tokio::spawn(async move {
+                            if let Err(e) =
+                                handle_workflow_event(&event, &db, &event_bus).await
+                            {
+                                tracing::warn!(
+                                    event_type = %event.event_type,
+                                    error = %e,
+                                    "Failed to handle workflow event for messages"
+                                );
+                            }
+                        });
+                    }
+                    Err(tokio::sync::broadcast::error::RecvError::Lagged(n)) => {
+                        tracing::warn!(skipped = n, "Event listener lagged, skipping events");
+                    }
+                    Err(tokio::sync::broadcast::error::RecvError::Closed) => {
+                        tracing::info!("Event bus closed, stopping message event listener");
+                        break;
+                    }
+                }
+            }
+        });
+    }
 }
 
 impl Default for MessageModule {
@@ -97,3 +134,46 @@ impl ErpModule for MessageModule {
         self
     }
 }
+
+/// 处理工作流事件，生成对应的消息通知。
+async fn handle_workflow_event(
+    event: &erp_core::events::DomainEvent,
+    db: &sea_orm::DatabaseConnection,
+    event_bus: &EventBus,
+) -> Result<(), String> {
+    match event.event_type.as_str() {
+        "process_instance.started" => {
+            let instance_id = event.payload.get("instance_id")
+                .and_then(|v| v.as_str())
+                .unwrap_or("unknown");
+            let starter_id = event.payload.get("started_by")
+                .and_then(|v| v.as_str());
+
+            if let Some(starter) = starter_id {
+                let recipient = match uuid::Uuid::parse_str(starter) {
+                    Ok(id) => id,
+                    Err(_) => return Ok(()),
+                };
+                let _ = crate::service::message_service::MessageService::send_system(
+                    event.tenant_id,
+                    recipient,
+                    "流程已启动".to_string(),
+                    format!("您的流程实例 {} 已启动执行。", instance_id),
+                    "normal",
+                    Some("workflow_instance".to_string()),
+                    uuid::Uuid::parse_str(instance_id).ok(),
+                    db,
+                    event_bus,
+                )
+                .await
+                .map_err(|e| e.to_string())?;
+            }
+        }
+        "task.completed" => {
+            // 任务完成时通知发起人（此处简化处理）
+            tracing::debug!("Task completed event received, skipping notification for now");
+        }
+        _ => {}
+    }
+    Ok(())
+}