Implement global search

[Duhl-Liamz]

Closes #3

Implements a focused global search feature for the $5 issue.

Summary

• Adds authenticated GET /api/v1/search/ for resources visible to the current user
• Searches item title/description for regular users and also user email/full name for superusers
• Returns a stable, paginated result list with a true total count
• Adds a debounced navbar search dropdown with loading, empty, and error states
• Adds backend coverage for superuser results, normal-user scoping, result limits/counts, and blank queries

Why this version

This takes the same basic direction as the existing open PR, but addresses the review concerns up front:

• Debounces the navbar query and uses React Query cache timing instead of requesting on every keystroke
• Orders database results by id descending so result ordering is deterministic
• Makes count represent all matching records, not just the returned page
• Avoids brittle exact-array assertions in the normal-user access test

Verification

• python -m compileall backend\app
• python -m alembic upgrade head
• python -m pytest app\tests\api\routes\test_search.py -q (4 passed)
• python -m pytest app\tests -q -k "not test_reset_password and not pre_start" (55 passed, 4 deselected)
• python -m ruff check app
• npm ci
• npm run build
• biome check --no-errors-on-unmatched --files-ignore-unknown=true src\components\Common\Navbar.tsx

Local setup used portable PostgreSQL 16 on localhost:5432 plus portable Node.js 20/npm for the frontend build.

Note: the unfiltered backend suite currently has unrelated baseline failures: test_reset_password changes the superuser password without restoring it, and two pre-start tests use called_once_with instead of assert_called_once_with.