feat: include main_image_url in model fillable and query service SELECT

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Summary / 概要

Surface main_image_url from the Eloquent model and the read paths so downstream layers can consume it.

Changes

• Add main_image_url to WorldHeritage::$fillable.
• Add 'world_heritage_sites.main_image_url' to SELECT in:
  • WorldHeritageReadQueryService::findByIdsPreserveOrder
  • WorldHeritageQueryService::getAllHeritages
  • WorldHeritageQueryService::getHeritageById（with-clause / select 全箇所）
• add WorldHeritage::$fillable に main_image_url
• Read 系3箇所の SELECT に world_heritage_sites.main_image_url