feat(server): outbox relay 改为 LISTEN/NOTIFY + 30s 兜底轮询

- EventBus::publish() 持久化后执行 NOTIFY outbox_channel
- outbox relay 使用 sqlx::PgListener 监听 + tokio::select! 竞争
- 30s 兜底轮询防止 NOTIFY 丢失，断线自动重连
- 轮询间隔从 5s 提升到 30s，事件延迟降至 <100ms

Cargo.toml

@@ -38,6 +38,7 @@ sea-orm = { version = "1.1", features = [
     "sqlx-postgres", "runtime-tokio-rustls", "macros", "with-uuid", "with-chrono", "with-json"
 ] }
 sea-orm-migration = { version = "1.1", features = ["sqlx-postgres", "runtime-tokio-rustls"] }
+sqlx = { version = "0.8", features = ["runtime-tokio-rustls", "postgres", "uuid"] }
 
 # Serialization
 serde = { version = "1", features = ["derive"] }