Add multi-year budget-window API v1 support

[anth-volk]

Summary

• add API v1 budget-window economy routing and OpenAPI coverage
• adapt economy service tracking to the simulation API batch interface
• harden reform-impact tracking, provisional claims, schema handling, and simulation listing queries
• add focused unit and route coverage for the multi-year API v1 path

This is a clean PR branch based directly on current master, without using the #3387 branch history.

Tests

• FLASK_DEBUG=1 uv run pytest tests/unit/libs/test_simulation_api_modal.py tests/unit/services/test_economy_service.py tests/unit/services/test_reform_impacts_service.py tests/unit/endpoints/test_simulation.py tests/to_refactor/python/test_economy_budget_window_routes.py
• uv run ruff format --check .
• uv run ruff check policyengine_api/services/economy_service.py policyengine_api/services/reform_impacts_service.py policyengine_api/endpoints/simulation.py tests/unit/services/test_economy_service.py tests/unit/endpoints/test_simulation.py
• python -m py_compile policyengine_api/services/economy_service.py policyengine_api/services/reform_impacts_service.py policyengine_api/endpoints/simulation.py