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/dto.rs

@@ -0,0 +1,211 @@
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
+use utoipa::ToSchema;
+use uuid::Uuid;
+use validator::Validate;
+
+// --- 流程图节点/边定义 ---
+
+/// BPMN 节点类型
+#[derive(Debug, Clone, Serialize, Deserialize, ToSchema, PartialEq)]
+#[serde(rename_all = "camelCase")]
+pub enum NodeType {
+    StartEvent,
+    EndEvent,
+    UserTask,
+    ServiceTask,
+    ExclusiveGateway,
+    ParallelGateway,
+}
+
+/// 流程图节点定义
+#[derive(Debug, Clone, Serialize, Deserialize, ToSchema)]
+#[serde(rename_all = "camelCase")]
+pub struct NodeDef {
+    pub id: String,
+    #[serde(rename = "type")]
+    pub node_type: NodeType,
+    pub name: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub assignee_id: Option<Uuid>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub candidate_groups: Option<Vec<String>>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub service_type: Option<String>,
+    /// 前端渲染位置
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub position: Option<NodePosition>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, ToSchema)]
+#[serde(rename_all = "camelCase")]
+pub struct NodePosition {
+    pub x: f64,
+    pub y: f64,
+}
+
+/// 流程图连线定义
+#[derive(Debug, Clone, Serialize, Deserialize, ToSchema)]
+#[serde(rename_all = "camelCase")]
+pub struct EdgeDef {
+    pub id: String,
+    pub source: String,
+    pub target: String,
+    /// 条件表达式（排他网关分支）
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub condition: Option<String>,
+    /// 前端渲染标签
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub label: Option<String>,
+}
+
+/// 完整流程图
+#[derive(Debug, Clone, Serialize, Deserialize, ToSchema)]
+#[serde(rename_all = "camelCase")]
+pub struct FlowDiagram {
+    pub nodes: Vec<NodeDef>,
+    pub edges: Vec<EdgeDef>,
+}
+
+// --- 流程定义 DTOs ---
+
+#[derive(Debug, Serialize, ToSchema)]
+pub struct ProcessDefinitionResp {
+    pub id: Uuid,
+    pub name: String,
+    pub key: String,
+    pub version: i32,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub category: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub description: Option<String>,
+    pub nodes: serde_json::Value,
+    pub edges: serde_json::Value,
+    pub status: String,
+    pub created_at: DateTime<Utc>,
+    pub updated_at: DateTime<Utc>,
+}
+
+#[derive(Debug, Deserialize, Validate, ToSchema)]
+pub struct CreateProcessDefinitionReq {
+    #[validate(length(min = 1, max = 200, message = "流程名称不能为空"))]
+    pub name: String,
+    #[validate(length(min = 1, max = 100, message = "流程编码不能为空"))]
+    pub key: String,
+    pub category: Option<String>,
+    pub description: Option<String>,
+    pub nodes: Vec<NodeDef>,
+    pub edges: Vec<EdgeDef>,
+}
+
+#[derive(Debug, Deserialize, ToSchema)]
+pub struct UpdateProcessDefinitionReq {
+    pub name: Option<String>,
+    pub category: Option<String>,
+    pub description: Option<String>,
+    pub nodes: Option<Vec<NodeDef>>,
+    pub edges: Option<Vec<EdgeDef>>,
+}
+
+// --- 流程实例 DTOs ---
+
+#[derive(Debug, Serialize, ToSchema)]
+pub struct ProcessInstanceResp {
+    pub id: Uuid,
+    pub definition_id: Uuid,
+    pub definition_name: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub business_key: Option<String>,
+    pub status: String,
+    pub started_by: Uuid,
+    pub started_at: DateTime<Utc>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub completed_at: Option<DateTime<Utc>>,
+    pub created_at: DateTime<Utc>,
+    /// 当前活跃的 token 位置
+    pub active_tokens: Vec<TokenResp>,
+}
+
+#[derive(Debug, Deserialize, ToSchema)]
+pub struct StartInstanceReq {
+    pub definition_id: Uuid,
+    pub business_key: Option<String>,
+    /// 初始流程变量
+    pub variables: Option<Vec<SetVariableReq>>,
+}
+
+// --- Token DTOs ---
+
+#[derive(Debug, Serialize, ToSchema)]
+pub struct TokenResp {
+    pub id: Uuid,
+    pub node_id: String,
+    pub status: String,
+    pub created_at: DateTime<Utc>,
+}
+
+// --- 任务 DTOs ---
+
+#[derive(Debug, Serialize, ToSchema)]
+pub struct TaskResp {
+    pub id: Uuid,
+    pub instance_id: Uuid,
+    pub token_id: Uuid,
+    pub node_id: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub node_name: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub assignee_id: Option<Uuid>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub candidate_groups: Option<serde_json::Value>,
+    pub status: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub outcome: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub form_data: Option<serde_json::Value>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub due_date: Option<DateTime<Utc>>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub completed_at: Option<DateTime<Utc>>,
+    pub created_at: DateTime<Utc>,
+    /// 流程定义名称（用于列表展示）
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub definition_name: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub business_key: Option<String>,
+}
+
+#[derive(Debug, Deserialize, ToSchema)]
+pub struct CompleteTaskReq {
+    pub outcome: String,
+    pub form_data: Option<serde_json::Value>,
+}
+
+#[derive(Debug, Deserialize, ToSchema)]
+pub struct DelegateTaskReq {
+    pub delegate_to: Uuid,
+}
+
+// --- 流程变量 DTOs ---
+
+#[derive(Debug, Serialize, ToSchema)]
+pub struct ProcessVariableResp {
+    pub id: Uuid,
+    pub name: String,
+    pub var_type: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub value_string: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub value_number: Option<f64>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub value_boolean: Option<bool>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub value_date: Option<DateTime<Utc>>,
+}
+
+#[derive(Debug, Clone, Deserialize, ToSchema)]
+pub struct SetVariableReq {
+    pub name: String,
+    pub var_type: Option<String>,
+    pub value: serde_json::Value,
+}