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/handler/instance_handler.rs

@@ -0,0 +1,112 @@
+use axum::Extension;
+use axum::extract::{FromRef, Path, Query, State};
+use axum::response::Json;
+
+use erp_core::error::AppError;
+use erp_core::rbac::require_permission;
+use erp_core::types::{ApiResponse, PaginatedResponse, Pagination, TenantContext};
+use uuid::Uuid;
+
+use crate::dto::{ProcessInstanceResp, StartInstanceReq};
+use crate::service::instance_service::InstanceService;
+use crate::workflow_state::WorkflowState;
+
+/// POST /api/v1/workflow/instances
+pub async fn start_instance<S>(
+    State(state): State<WorkflowState>,
+    Extension(ctx): Extension<TenantContext>,
+    Json(req): Json<StartInstanceReq>,
+) -> Result<Json<ApiResponse<ProcessInstanceResp>>, AppError>
+where
+    WorkflowState: FromRef<S>,
+    S: Clone + Send + Sync + 'static,
+{
+    require_permission(&ctx, "workflow:start")?;
+
+    let resp = InstanceService::start(
+        ctx.tenant_id,
+        ctx.user_id,
+        &req,
+        &state.db,
+        &state.event_bus,
+    )
+    .await?;
+
+    Ok(Json(ApiResponse::ok(resp)))
+}
+
+/// GET /api/v1/workflow/instances
+pub async fn list_instances<S>(
+    State(state): State<WorkflowState>,
+    Extension(ctx): Extension<TenantContext>,
+    Query(pagination): Query<Pagination>,
+) -> Result<Json<ApiResponse<PaginatedResponse<ProcessInstanceResp>>>, AppError>
+where
+    WorkflowState: FromRef<S>,
+    S: Clone + Send + Sync + 'static,
+{
+    require_permission(&ctx, "workflow:list")?;
+
+    let (instances, total) =
+        InstanceService::list(ctx.tenant_id, &pagination, &state.db).await?;
+
+    let page = pagination.page.unwrap_or(1);
+    let page_size = pagination.limit();
+    let total_pages = (total + page_size - 1) / page_size;
+
+    Ok(Json(ApiResponse::ok(PaginatedResponse {
+        data: instances,
+        total,
+        page,
+        page_size,
+        total_pages,
+    })))
+}
+
+/// GET /api/v1/workflow/instances/{id}
+pub async fn get_instance<S>(
+    State(state): State<WorkflowState>,
+    Extension(ctx): Extension<TenantContext>,
+    Path(id): Path<Uuid>,
+) -> Result<Json<ApiResponse<ProcessInstanceResp>>, AppError>
+where
+    WorkflowState: FromRef<S>,
+    S: Clone + Send + Sync + 'static,
+{
+    require_permission(&ctx, "workflow:read")?;
+
+    let resp = InstanceService::get_by_id(id, ctx.tenant_id, &state.db).await?;
+    Ok(Json(ApiResponse::ok(resp)))
+}
+
+/// POST /api/v1/workflow/instances/{id}/suspend
+pub async fn suspend_instance<S>(
+    State(state): State<WorkflowState>,
+    Extension(ctx): Extension<TenantContext>,
+    Path(id): Path<Uuid>,
+) -> Result<Json<ApiResponse<()>>, AppError>
+where
+    WorkflowState: FromRef<S>,
+    S: Clone + Send + Sync + 'static,
+{
+    require_permission(&ctx, "workflow:update")?;
+
+    InstanceService::suspend(id, ctx.tenant_id, ctx.user_id, &state.db).await?;
+    Ok(Json(ApiResponse::ok(())))
+}
+
+/// POST /api/v1/workflow/instances/{id}/terminate
+pub async fn terminate_instance<S>(
+    State(state): State<WorkflowState>,
+    Extension(ctx): Extension<TenantContext>,
+    Path(id): Path<Uuid>,
+) -> Result<Json<ApiResponse<()>>, AppError>
+where
+    WorkflowState: FromRef<S>,
+    S: Clone + Send + Sync + 'static,
+{
+    require_permission(&ctx, "workflow:update")?;
+
+    InstanceService::terminate(id, ctx.tenant_id, ctx.user_id, &state.db).await?;
+    Ok(Json(ApiResponse::ok(())))
+}