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-server/Cargo.toml

@@ -29,3 +29,4 @@ erp-workflow.workspace = true
 erp-message.workspace = true
 anyhow.workspace = true
 uuid.workspace = true
+chrono.workspace = true