feat(household): add shared household budgeting support

[yuliuyi717-ux]

Closes #134

Summary

• add household domain (households, household_members) with invite-based create/join/leave/member-remove flows
• allow household-scoped categories, expenses, and bills with access checks for shared collaboration
• update dashboard aggregation and AI insights scope to include authorized household records
• add docs + OpenAPI updates and integration tests for household collaboration paths

Validation

• sh ./scripts/test-backend.sh tests/test_households.py
• sh ./scripts/test-backend.sh tests/test_categories.py tests/test_expenses.py tests/test_bills.py tests/test_dashboard.py

[yuliuyi717-ux]

Quick check-in for this PR.

Implementation is ready from my side and I’m waiting on maintainer review.

Please let me know if you want any adjustments and I’ll patch promptly.