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/workflowTasks.ts

@@ -0,0 +1,61 @@
+import client from './client';
+import type { PaginatedResponse } from './users';
+
+export interface TaskInfo {
+  id: string;
+  instance_id: string;
+  token_id: string;
+  node_id: string;
+  node_name?: string;
+  assignee_id?: string;
+  candidate_groups?: unknown;
+  status: string;
+  outcome?: string;
+  form_data?: unknown;
+  due_date?: string;
+  completed_at?: string;
+  created_at: string;
+  definition_name?: string;
+  business_key?: string;
+}
+
+export interface CompleteTaskRequest {
+  outcome: string;
+  form_data?: Record<string, unknown>;
+}
+
+export interface DelegateTaskRequest {
+  delegate_to: string;
+}
+
+export async function listPendingTasks(page = 1, pageSize = 20) {
+  const { data } = await client.get<{ success: boolean; data: PaginatedResponse<TaskInfo> }>(
+    '/workflow/tasks/pending',
+    { params: { page, page_size: pageSize } },
+  );
+  return data.data;
+}
+
+export async function listCompletedTasks(page = 1, pageSize = 20) {
+  const { data } = await client.get<{ success: boolean; data: PaginatedResponse<TaskInfo> }>(
+    '/workflow/tasks/completed',
+    { params: { page, page_size: pageSize } },
+  );
+  return data.data;
+}
+
+export async function completeTask(id: string, req: CompleteTaskRequest) {
+  const { data } = await client.post<{ success: boolean; data: TaskInfo }>(
+    `/workflow/tasks/${id}/complete`,
+    req,
+  );
+  return data.data;
+}
+
+export async function delegateTask(id: string, req: DelegateTaskRequest) {
+  const { data } = await client.post<{ success: boolean; data: TaskInfo }>(
+    `/workflow/tasks/${id}/delegate`,
+    req,
+  );
+  return data.data;
+}