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-message/src/service/message_service.rs

@@ -130,7 +130,7 @@ impl MessageService {
                 "recipient_id": req.recipient_id,
                 "title": req.title,
             }),
-        ));
+        ), db).await;
 
         audit_service::record(
             AuditLog::new(tenant_id, Some(sender_id), "message.send", "message")
@@ -198,7 +198,7 @@ impl MessageService {
                 "message_id": id,
                 "recipient_id": recipient_id,
             }),
-        ));
+        ), db).await;
 
         audit_service::record(
             AuditLog::new(tenant_id, Some(system_user), "message.send_system", "message")