Thumbnail image and map pin can represent different countries for transnational/serial properties

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Description

For transnational/serial World Heritage properties (one property spread across multiple countries), the thumbnail image and the map pin shown on the detail page can depict different countries, which is confusing.

Example: id 1321 ("The Architectural Work of Le Corbusier...", 7 countries: ARG, BEL, CHE, DEU, FRA, IND, JPN).

• thumbnail_url (main_image_url) → https://whc.unesco.org/document/140720, which depicts the Curutchet House in La Plata, Argentina.
• latitude/longitude → 46.4684139, 6.8293361, which points to Switzerland.
• country / primary_state_party_code → India (IND, marked is_primary: true).

So the same detail page shows three different countries depending on which field you look at (thumbnail → Argentina, map → Switzerland, country label → India).

Root cause (API side)

UNESCO's own dataset stores exactly one (latitude, longitude) pair per heritage site record, even for serial/transnational properties that span many countries and many physical locations. main_image_url/per-image data is independent of that single coordinate, so there's no guarantee they refer to the same sub-site or country. This is a data modeling limitation, not just a rendering bug — see world-heritage-api's world_heritage_sites table (single latitude/longitude columns) and WorldHeritageQueryService::getHeritageById.

Suggested direction (needs decision before implementing)

1. Quick/cosmetic fix: when a property has multiple state_party_codes, make both the thumbnail and the map consistently represent the primary_state_party_code (here, India) — i.e. prefer an image tagged/known to be from the primary country, and keep using the stored lat/lng but label it clearly (e.g. "Representative location: India" or similar), or
2. Proper fix: extend the data model so each image (and ideally each sub-site) can carry its own country/coordinate, and let the frontend pick a coordinate that matches whichever image/country is currently displayed.

Related: #124 (single source of truth for image rendering), #127 (frontend rendering rules for thumbnail/gallery).

Acceptance Criteria

• Decide whether the fix lives in world-heritage-api (data/schema), world-heritage-frontend (rendering), or both.
• For a given heritage detail page, the country implied by the thumbnail and the country implied by the map pin are consistent with each other (and ideally with country/primary_state_party_code).
• Verify against id 1321 and audit other multi-country serial properties for the same mismatch.