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

apps/web/src/api/workflowDefinitions.ts

@@ -0,0 +1,89 @@
+import client from './client';
+import type { PaginatedResponse } from './users';
+
+export interface NodeDef {
+  id: string;
+  type: 'StartEvent' | 'EndEvent' | 'UserTask' | 'ServiceTask' | 'ExclusiveGateway' | 'ParallelGateway';
+  name: string;
+  assignee_id?: string;
+  candidate_groups?: string[];
+  service_type?: string;
+  position?: { x: number; y: number };
+}
+
+export interface EdgeDef {
+  id: string;
+  source: string;
+  target: string;
+  condition?: string;
+  label?: string;
+}
+
+export interface ProcessDefinitionInfo {
+  id: string;
+  name: string;
+  key: string;
+  version: number;
+  category?: string;
+  description?: string;
+  nodes: NodeDef[];
+  edges: EdgeDef[];
+  status: string;
+  created_at: string;
+  updated_at: string;
+}
+
+export interface CreateProcessDefinitionRequest {
+  name: string;
+  key: string;
+  category?: string;
+  description?: string;
+  nodes: NodeDef[];
+  edges: EdgeDef[];
+}
+
+export interface UpdateProcessDefinitionRequest {
+  name?: string;
+  category?: string;
+  description?: string;
+  nodes?: NodeDef[];
+  edges?: EdgeDef[];
+}
+
+export async function listProcessDefinitions(page = 1, pageSize = 20) {
+  const { data } = await client.get<{ success: boolean; data: PaginatedResponse<ProcessDefinitionInfo> }>(
+    '/workflow/definitions',
+    { params: { page, page_size: pageSize } },
+  );
+  return data.data;
+}
+
+export async function getProcessDefinition(id: string) {
+  const { data } = await client.get<{ success: boolean; data: ProcessDefinitionInfo }>(
+    `/workflow/definitions/${id}`,
+  );
+  return data.data;
+}
+
+export async function createProcessDefinition(req: CreateProcessDefinitionRequest) {
+  const { data } = await client.post<{ success: boolean; data: ProcessDefinitionInfo }>(
+    '/workflow/definitions',
+    req,
+  );
+  return data.data;
+}
+
+export async function updateProcessDefinition(id: string, req: UpdateProcessDefinitionRequest) {
+  const { data } = await client.put<{ success: boolean; data: ProcessDefinitionInfo }>(
+    `/workflow/definitions/${id}`,
+    req,
+  );
+  return data.data;
+}
+
+export async function publishProcessDefinition(id: string) {
+  const { data } = await client.post<{ success: boolean; data: ProcessDefinitionInfo }>(
+    `/workflow/definitions/${id}/publish`,
+  );
+  return data.data;
+}