Add stored household CRUD support

[anth-volk]

Summary

Add the ability to store and retrieve household definitions that can be reused across calculations and impact analyses.

Scope

Models

• Household model with:
  • tax_benefit_model_name (policyengine_uk or policyengine_us)
  • year
  • label (optional)
  • household_data JSONB containing people and entity groups

Endpoints

• POST /households - Create a stored household
• GET /households - List households with optional filtering
• GET /households/{id} - Get a household by ID
• DELETE /households/{id} - Delete a household

Database

• Migration: 20260203000000_create_households.sql

Acceptance Criteria

• Household model created with all fields
• CRUD endpoints working
• Tests passing
• Migration applied successfully