feat: persist main_image_url via import command

[zigzagdev]

Summary / 概要

Update ImportWorldHeritageSiteFromSplitFile (or whichever import command consumes world_heritage_sites.json) to write main_image_url into the new column.

Changes / 対応内容

• Map main_image_url from JSON row to the model attribute on insert/upsert.
• Ensure null-safe (column allows null).