Add global search

[Ninot1Quyi]

Summary

• add a backend /api/v1/search endpoint that searches visible items and, for superusers, users
• wire the navbar search input to the new endpoint
• show typed result cards with navigation to Items or Admin

Fixes #3

Checks

• npm run build
• npx biome check src/components/Common/Navbar.tsx src/client/models.ts src/client/services.ts
• poetry run ruff check app/api/routes/search.py app/models.py app/tests/api/routes/test_search.py
• poetry run mypy app/api/routes/search.py app/models.py
• direct search smoke test with sqlite

Note: pytest target tests require a local Postgres role/database matching the project .env; this machine did not have role "postgres" available.