feat: add geoformat page model with coverFiles method

Expose combined files (page files + map children files) for the cover
field query in the geoformat blueprint.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/site/models/geoformat.php

@@ -0,0 +1,13 @@
+<?php
+
+use Kirby\Cms\Page;
+
+class GeoformatPage extends Page
+{
+    public function coverFiles()
+    {
+        return $this->files()->add(
+            $this->children()->filterBy('intendedTemplate', 'map')->files()
+        );
+    }
+}