Add global search

[RSimmons2021]

Summary

Fixes #3

• Adds an authenticated /api/v1/search/ endpoint for records visible to the current user.
• Searches items for all users and includes user results for superusers.
• Adds a sidebar global search input that links results back to Items or Admin.
• Adds backend route tests for visible item results, permission filtering, and superuser user results.

Validation

• npm ci
• npm run build
• python -m compileall backend/app
• git diff --check

Backend pytest was not run locally because Poetry could not install the locked backend dependencies on the available Python 3.13 runtime; the install stops before pytest is available.