Fix Modal gateway routing metadata imports

[anth-volk]

Fixes #1543

Summary

• Moved Modal routing metadata helpers out of policyengine_household_api.utils and into policyengine_household_api.modal_release.
• Updated gateway, worker dispatch, analytics, and related tests to import from the side-effect-free Modal release module.
• Added an inline comment documenting why the gateway keeps channel values as a literal instead of importing the app enum.

Testing

• uv run pytest tests/unit/modal_release/test_gateway.py tests/unit/modal_release/test_worker_dispatch.py -q
• uv run pytest tests/unit/endpoints/test_household.py::TestCalculateEndpoint::test__analytics_enabled__captures_modal_routing_metadata -q
• make format-check

Note: make format-check passed after printing the existing Docker daemon warning from Makefile variable expansion.