♻️ (seaorm): Adapta al arranque async

- Elimina `run_now` y convierte `initialize()` en `async fn`.
- `DBCONN` pasa de `LazyLock` a `OnceLock`; `MigratorBase` adopta
  `#[pagetop::async_trait]` con métodos async nativos.

extensions/pagetop-seaorm/src/migration/schema.rs

@@ -15,7 +15,7 @@
 //!
 //! pub struct Migration;
 //!
-//! #[async_trait::async_trait]
+//! #[pagetop::async_trait]
 //! impl MigrationTrait for Migration {
 //!     async fn up(&self, manager: &SchemaManager) -> Result<(), DbErr> {
 //!         let table = table_auto(Users::Table)