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

Cargo.lock

@@ -898,6 +898,7 @@ dependencies = [
  "erp-config",
  "erp-core",
  "erp-server-migration",
+ "erp-workflow",
  "redis",
  "sea-orm",
  "serde",
@@ -924,15 +925,19 @@ name = "erp-workflow"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "async-trait",
  "axum",
  "chrono",
  "erp-core",
  "sea-orm",
  "serde",
  "serde_json",
+ "thiserror",
  "tokio",
  "tracing",
+ "utoipa",
  "uuid",
+ "validator",
 ]
 
 [[package]]