Implement global search

[sonson0910]

Closes #3

Summary

• Add authenticated GET /api/v1/search/ for resources visible to the current user
• Search item title/description for regular users and search users too for superusers
• Add a debounced navbar search dropdown with loading and empty states
• Add frontend client types/service and focused backend route coverage

Verification

• npm run build
• python -m compileall app
• ruff check app/api/routes/search.py app/tests/api/routes/test_search.py app/api/main.py app/models.py
• Direct SQLite function check for normal-user scoping and superuser user search

Note: full pytest requires a local PostgreSQL instance at localhost:5432. Docker was installed but the Docker daemon was not running in this environment.