✨ (admin): Añade la extensión pagetop-admin

Panel de administración extensible: registro de secciones/páginas/
tareas/acciones vía acciones `Declare*`, formularios de configuración
automáticos (ConfigForm + SettingsSchema) y persistencia en la tabla
`settings`.

extensions/pagetop-admin/src/migration/m20260629_000001_create_settings.rs

@@ -0,0 +1,49 @@
+use pagetop_seaorm::migration::*;
+
+/// Tabla `settings`: almacén clave-valor JSON para configuración persistente.
+pub struct Migration;
+
+#[pagetop::async_trait]
+impl MigrationTrait for Migration {
+    async fn up(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+        manager
+            .create_table(
+                Table::create()
+                    .table(Settings::Table)
+                    .if_not_exists()
+                    .col(string_len(Settings::Key, 190).primary_key())
+                    .col(string_len(Settings::Scope, 64))
+                    .col(text(Settings::Value))
+                    .col(timestamp(Settings::UpdatedAt).default(Expr::current_timestamp()))
+                    .col(integer_null(Settings::UpdatedBy))
+                    .to_owned(),
+            )
+            .await?;
+
+        manager
+            .create_index(
+                Index::create()
+                    .name("idx_settings_scope")
+                    .table(Settings::Table)
+                    .col(Settings::Scope)
+                    .to_owned(),
+            )
+            .await
+    }
+
+    async fn down(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+        manager
+            .drop_table(Table::drop().table(Settings::Table).to_owned())
+            .await
+    }
+}
+
+#[derive(DeriveIden)]
+enum Settings {
+    Table,
+    Key,
+    Scope,
+    Value,
+    UpdatedAt,
+    UpdatedBy,
+}