Remove Modal custom domain wiring and pin core

[anth-volk]

Fixes #1523

Summary

• Remove Modal gateway custom-domain registration so deploys do not try to claim the production URL before cutover.
• Keep the stable Modal gateway endpoint label for generated Modal URLs.
• Add an explicit policyengine-core>=3.26.5 dependency and refresh uv.lock.
• Add a Towncrier changelog fragment.

Local checks

• uv run --extra dev pytest -vv --timeout=150 -rP tests/unit/modal_release/test_gateway_app.py
• make format-check
• uv run ruff check policyengine_household_api/modal_release/gateway_app.py tests/unit/modal_release/test_gateway_app.py
• uv run ruff check . still fails on existing unrelated repo-wide lint issues; changed files pass.

[anth-volk]

Note: temporary fix to allow local testing