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.

Cargo.lock

@@ -896,6 +896,7 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "axum",
+ "chrono",
  "config",
  "erp-auth",
  "erp-common",