Remove household_options from the country metadata #4209

Problem

#4159 added household_options.time_period to the /{country}/metadata response: a list of years for household calculations, separate from economy_options.time_period. No client reads it.

The maintainer decided on 4–5 November 2026 that household and society-wide calculations share one list of years per country. A separate household list is therefore no longer needed.

Change

Remove what #4159 added:

• household_options in the /{country}/metadata response, built by build_household_options in policyengine_api/country.py;
• policyengine_api/household_years.py;
• certified_spm_forecast_years in policyengine_api/spm.py, which only build_household_options used;
• their tests.

A follow-up change defines each country's years once and uses them for economy_options.time_period.