feat(core): implement event outbox persistence

Add domain_events migration and SeaORM entity. Modify EventBus::publish
to persist events before broadcasting (best-effort: DB failure logs
warning but still broadcasts in-memory). Update all 19 publish call
sites across 4 crates to pass db reference.

Add outbox relay background task that polls pending events every 5s
and re-broadcasts them, ensuring no events are lost on server restart.

crates/erp-workflow/src/service/task_service.rs

@@ -242,7 +242,7 @@ impl TaskService {
             "task.completed",
             tenant_id,
             serde_json::json!({ "task_id": id, "outcome": req.outcome }),
-        ));
+        ), db).await;
 
         audit_service::record(
             AuditLog::new(tenant_id, Some(operator_id), "task.complete", "task")