feat: push main_image_url through Algolia indexer

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Summary

Update AlgoliaImportWorldHeritages so each Algolia object carries main_image_url, and switch the existing thumbnail_url to derive from main_image_url first.

Changes / 対応内容

• Add 'world_heritage_sites.main_image_url' to the SELECT.
• Add 'main_image_url' => $row->main_image_url to the Algolia object.
• Change 'thumbnail_url' => $row->main_image_url ?? $row->images->first()?->url (preserve existing fallback).