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-server/migration/src/lib.rs

@@ -17,6 +17,11 @@ mod m20260412_000014_create_menus;
 mod m20260412_000015_create_menu_roles;
 mod m20260412_000016_create_settings;
 mod m20260412_000017_create_numbering_rules;
+mod m20260412_000018_create_process_definitions;
+mod m20260412_000019_create_process_instances;
+mod m20260412_000020_create_tokens;
+mod m20260412_000021_create_tasks;
+mod m20260412_000022_create_process_variables;
 
 pub struct Migrator;
 
@@ -41,6 +46,11 @@ impl MigratorTrait for Migrator {
             Box::new(m20260412_000015_create_menu_roles::Migration),
             Box::new(m20260412_000016_create_settings::Migration),
             Box::new(m20260412_000017_create_numbering_rules::Migration),
+            Box::new(m20260412_000018_create_process_definitions::Migration),
+            Box::new(m20260412_000019_create_process_instances::Migration),
+            Box::new(m20260412_000020_create_tokens::Migration),
+            Box::new(m20260412_000021_create_tasks::Migration),
+            Box::new(m20260412_000022_create_process_variables::Migration),
         ]
     }
 }