feat: prefer main_image_url for thumbnail_url in AlgoliaImportWorldHeritages

Switches the Algolia object's thumbnail_url to main_image_url first,
falling back to images[0]->url, so the search index mirrors the new
fallback semantics now used by WorldHeritageViewModel and
WorldHeritageDtoCollection::toSummaryArray.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: zigzagdev

src/app/Console/Commands/AlgoliaImportWorldHeritages.php

@@ -138,7 +138,7 @@ public function handle(): int
                         'year_inscribed' => $row->year_inscribed !== null ? (int) $row->year_inscribed : null,
                         'is_endangered' => (bool) $row->is_endangered,
                         'main_image_url' => $row->main_image_url,
-                        'thumbnail_url' => $row->images->first()?->url,
+                        'thumbnail_url' => $row->main_image_url ?? $row->images->first()?->url,
                         'state_party_codes' => $statePartyCodes,
                         'country_names_jp' => $countryCount > 1 ? $countryNamesJp : [],
                     ];