Epic: Test-coverage hardening

[untraceablez]

Goal: Bring the project in line with the constitution's Test-First Development (NON-NEGOTIABLE) principle. Today there are no real automated tests despite pytest scaffolding existing.

Current state

• Backend: backend/tests/conftest.py + utils.py exist (fixtures), but no actual test modules. Only ad-hoc backend/test_login.py, backend/test_setup.py.
• Frontend: no .test/.spec files at all.

Tasks

• Backend: contract tests for existing routers (auth, households, inventory, locations, allergens, barcode)
• Backend: service-layer unit tests
• Move/remove ad-hoc backend/test_login.py and backend/test_setup.py into the suite
• Frontend: Vitest unit/component tests for key components and services
• Frontend: Playwright E2E for the core flow (setup → login → add item → search)
• CI workflow to run backend + frontend tests on PRs

[untraceablez]

Kicked off: the backend test suite is now runnable for the first time (was scaffolding only) and a required backend-tests CI check gates main. 31 tests across unit/contract/integration. Remaining: broader backend coverage + frontend Vitest/Playwright.