chore: backfill main_image_url on existing world_heritage_sites rows

[zigzagdev]

Summary

One-shot SQL update to populate main_image_url on the 1248 existing rows from the raw UNESCO JSON, mirroring the approach used for is_endangered.

Changes

• Local: python3 -c "..." | docker exec -i heritage-mysql mysql -u world-heritage -pworld-heritage world_heritage_local style one-liner.
• Document the exact command in PR description so production can run it on Aiven post-deploy.
• No new artisan command (per project convention for one-off ops).