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/Cargo.toml

@@ -5,12 +5,16 @@ edition.workspace = true
 
 [dependencies]
 erp-core.workspace = true
-tokio.workspace = true
-serde.workspace = true
-serde_json.workspace = true
-uuid.workspace = true
-chrono.workspace = true
-axum.workspace = true
-sea-orm.workspace = true
-tracing.workspace = true
+tokio = { workspace = true, features = ["full"] }
+serde = { workspace = true, features = ["derive"] }
+serde_json = { workspace = true }
+uuid = { workspace = true, features = ["v7", "serde"] }
+chrono = { workspace = true, features = ["serde"] }
+axum = { workspace = true }
+sea-orm = { workspace = true, features = ["sqlx-postgres", "runtime-tokio-rustls", "with-uuid", "with-chrono", "with-json"] }
+tracing = { workspace = true }
 anyhow.workspace = true
+thiserror.workspace = true
+utoipa = { workspace = true, features = ["uuid", "chrono"] }
+async-trait.workspace = true
+validator.workspace = true