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/workflow_state.rs

@@ -0,0 +1,11 @@
+use erp_core::events::EventBus;
+use sea_orm::DatabaseConnection;
+
+/// Workflow-specific state extracted from the server's AppState via `FromRef`.
+///
+/// Contains the database connection and event bus needed by workflow handlers.
+#[derive(Clone)]
+pub struct WorkflowState {
+    pub db: DatabaseConnection,
+    pub event_bus: EventBus,
+}