Bug[🐛]: Fix UNESCO image import: store only images in world_heritage_site_images with stable ordering (no image_url writes)

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Goal

During the UNESCO import process, do not write image URLs into the main table fields (e.g. world_heritage_sites.image_url / primary_image_url).
Instead, persist all site images only into world_heritage_site_images, with stable sort_order and a clear is_primary rule, so the detail API can reliably return a non-null images array.