feat: restore transboundary flag as is_transboundary column / 越境遺産フラグの復活

[zigzagdev]

Motivation / 目的

UNESCO's own dataset already flags whether a site is transboundary (spans multiple countries via a serial nomination), but this project fetched that value and then silently discarded it during import. This restores it as a real column.

UNESCOのデータセット自体が「越境遺産かどうか」を示す真偽値を既に持っていますが、これまでは取得しておきながらインポート処理の途中で捨てていました。実カラムとして復活させます。

What I have done / 実施内容

• Added is_transboundary (boolean, default false) column to world_heritage_sites via migration
• SplitWorldHeritageJson::normalizeSiteRowImportReady() / mergeSiteRowPreferExisting() now carry transboundary from the raw UNESCO dump through to the normalized site record (same pattern as the existing is_endangered handling)
• ImportWorldHeritageSiteFromSplitFile now persists is_transboundary to the DB
• Added is_transboundary to WorldHeritage::$fillable
• Re-shipped the checked-in storage/app/private/unesco/normalized/world_heritage_sites.json snapshot with is_transboundary populated for all 1248 sites (same precedent as the earlier main_image_url backfill)

Verification / 検証

Ran the full pipeline (split → import) end to end against the real UNESCO dump and confirmed in the DB:

• Known transnational serial sites (1133 Ancient and Primeval Beech Forests, 1321 The Architectural Work of Le Corbusier, 1363 Prehistoric Pile Dwellings around the Alps, 1613 The Great Spa Towns of Europe) → is_transboundary = true
• A single-country site (1000) → is_transboundary = false

Full WorldHeritage-related test suite (63 tests) re-run with no regressions.

Scope / スコープ

This PR stops at the DB layer as agreed. Exposing is_transboundary via the API/DTO or as an Algolia facet is intentionally out of scope here and will be a separate follow-up once a concrete feature (e.g. a transnational-sites filter) needs it.

Closes #581

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.02%. Comparing base (905138c) to head (62d7f01).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #582      +/-   ##
============================================
+ Coverage     64.66%   67.02%   +2.36%     
- Complexity     1694     1707      +13     
============================================
  Files           147      149       +2     
  Lines          8858     8929      +71     
============================================
+ Hits           5728     5985     +257     
+ Misses         3130     2944     -186     

Files with missing lines	Coverage Δ
.../Commands/ImportWorldHeritageSiteFromSplitFile.php	73.19% <100.00%> (+73.19%)	⬆️
...rc/app/Console/Commands/SplitWorldHeritageJson.php	14.58% <100.00%> (+14.58%)	⬆️
src/app/Models/WorldHeritage.php	100.00% <100.00%> (ø)
...ldHeritageSiteFromSplitFileIsTransboundaryTest.php	100.00% <100.00%> (ø)
...ests/SplitWorldHeritageJsonIsTransboundaryTest.php	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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