feat: emit main_image_url from SplitWorldHeritageJson

[zigzagdev]

Summary

Update SplitWorldHeritageJson::normalizeSiteRowImportReady() (and its merge counterpart) to surface main_image_url in the produced world_heritage_sites.json.

Changes / 対応内容

• In normalizeSiteRowImportReady, add 'main_image_url' => $this->stringOrNull($row['main_image_url'] ?? null).
• In mergeSiteRowPreferExisting, fill main_image_url if existing is null/empty.

Acceptance

• php artisan world-heritage:split-json --in=... produces sites JSON entries with main_image_url.