Remove UK constituency map coordinates from impact responses

該当する API レスポンスモデル、シリアライザー、テスト、OpenAPI コントラクト、および issue で指定された API v2 の選挙区影響永続化モデルを、必要な Alembic マイグレーションとともに確認します。まず PolicyEngine/policyengine-app-v2#1190 の互換性要件を確認し、その後、現在の選挙区レスポンスと永続化のフィールドを追跡します。アイデンティティと影響値が維持され、コントラクトとストレージから座標がなくなり、コントラクトカバレッジがパスし、デプロイがフロントエンドの検証を待つ状態になれば完了です。

Supersedes PolicyEngine/policyengine-uk#1152 together with PolicyEngine/policyengine-app-v2#1190.

Current behavior

The UK constituency comparison code reads statistical weights from the private UK data repository and reads constituency codes, names, and hex-layout coordinates from the public UK data repository. It calculates constituency impacts, then places frontend-specific x and y positions in each API result. The planned API v2 ConstituencyImpact persistence model also stores those positions.

Required change

• Preserve constituency statistical aggregation and return the constituency identity, average household income change, and relative household income change without presentation coordinates.
• Use a stable constituency identifier in the response contract so app-v2 can join results to its versioned layout.
• Remove x and y from the applicable API response models, serializers, tests, OpenAPI contract, and API v2 constituency-impact persistence model.
• Add the required v2 Alembic migration if the coordinate columns have already been created in a retained target.
• Do not remove coordinates from a deployed response until PolicyEngine/policyengine-app-v2#1190 has been deployed and verified.
• Add contract coverage proving that constituency values and identities are preserved while layout positions are absent.

Deployment order

PolicyEngine/policyengine-app-v2#1190 is the compatibility prerequisite. Release the API contract change only after the frontend renders the same map from its own layout artifact.