Remove UK constituency map coordinates from impact responses

[anth-volk]

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 Own the UK constituency hex-map layout in the frontend 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.