feat(workflow): add workflow engine module (Phase 4)

Implement complete workflow engine with BPMN subset support:

Backend (erp-workflow crate):
- Token-driven execution engine with exclusive/parallel gateway support
- BPMN parser with flow graph validation
- Expression evaluator for conditional branching
- Process definition CRUD with draft/publish lifecycle
- Process instance management (start, suspend, terminate)
- Task service (pending, complete, delegate)
- PostgreSQL advisory locks for concurrent safety
- 5 database tables: process_definitions, process_instances,
  tokens, tasks, process_variables
- 13 API endpoints with RBAC protection
- Timeout checker framework (placeholder)

Frontend:
- Workflow page with 4 tabs (definitions, pending, completed, monitor)
- React Flow visual process designer (@xyflow/react)
- Process viewer with active node highlighting
- 3 API client modules for workflow endpoints
- Sidebar menu integration

crates/erp-workflow/src/engine/timeout.rs

@@ -0,0 +1,36 @@
+// 超时检查框架 — 占位实现
+//
+// 当前版本仅提供接口定义，实际超时检查逻辑将在后续迭代中实现。
+// Task 表的 due_date 字段已支持设置超时时间。
+
+use chrono::Utc;
+use sea_orm::{ColumnTrait, EntityTrait, QueryFilter};
+use uuid::Uuid;
+
+use crate::entity::task;
+use crate::error::WorkflowResult;
+
+/// 超时检查服务（占位）。
+pub struct TimeoutChecker;
+
+impl TimeoutChecker {
+    /// 查询已超时但未完成的任务列表。
+    ///
+    /// 返回 due_date < now 且 status = 'pending' 的任务 ID。
+    pub async fn find_overdue_tasks(
+        tenant_id: Uuid,
+        db: &sea_orm::DatabaseConnection,
+    ) -> WorkflowResult<Vec<Uuid>> {
+        let now = Utc::now();
+        let overdue = task::Entity::find()
+            .filter(task::Column::TenantId.eq(tenant_id))
+            .filter(task::Column::Status.eq("pending"))
+            .filter(task::Column::DueDate.lt(now))
+            .filter(task::Column::DeletedAt.is_null())
+            .all(db)
+            .await
+            .map_err(|e| crate::error::WorkflowError::Validation(e.to_string()))?;
+
+        Ok(overdue.iter().map(|t| t.id).collect())
+    }
+}