feat: add migration for main_image_url column on world_heritage_sites

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Summary

Create a migration that adds a nullable main_image_url text/string column to world_heritage_sites.

Changes

• New migration add_main_image_url_to_world_heritage_sites.
• Schema::table('world_heritage_sites', fn ($t) => $t->string('main_image_url', 2048)->nullable()) (or text if 2048 looks too long).
• Down migration drops the column.
• up: string('main_image_url', 2048)->nullable()
• down: drop column。

Acceptance

• Column exists, nullable, indexed schema sanity-checked.