♻️ (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/migrator.rs

@@ -57,7 +57,7 @@ impl Migration {
 }
 
 /// Performing migrations on a database
-#[async_trait::async_trait]
+#[pagetop::async_trait]
 pub trait MigratorTrait: Send {
     /// Vector of migrations in time sequence
     fn migrations() -> Vec<Box<dyn MigrationTrait>>;