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/src/main.rs

@@ -2,6 +2,7 @@ mod config;
 mod db;
 mod handlers;
 mod middleware;
+mod outbox;
 mod state;
 
 /// OpenAPI 规范定义（预留，未来可通过 utoipa derive 合并各模块 schema）。
@@ -142,6 +143,10 @@ async fn main() -> anyhow::Result<()> {
     erp_message::MessageModule::start_event_listener(db.clone(), event_bus.clone());
     tracing::info!("Message event listener started");
 
+    // Start outbox relay (re-publish pending domain events)
+    outbox::start_outbox_relay(db.clone(), event_bus.clone());
+    tracing::info!("Outbox relay started");
+
     let host = config.server.host.clone();
     let port = config.server.port;