Add search functionality to case listings

[darshacharya]

Description

Users cannot search for cases by keyword. The browse page has category and urgency filters but no text search.

Requirements

Backend

• Add a search query parameter to GET /api/cases
• Search should match against case title and description (case-insensitive)
• Also add to GET /api/admin/cases

Frontend

• Add a search input at the top of the cases browse page (frontend/src/app/cases/page.tsx)
• Debounce the search input (300ms) to avoid excessive API calls
• Add search to the admin dashboard list (frontend/src/app/admin/page.tsx)
• Show "No results for 'query'" when search returns empty

Tech Notes

• Backend: Use SQLAlchemy's ilike for case-insensitive search
• Frontend: Use useEffect with a debounce timer or a useDebouncedValue hook

Acceptance Criteria

• Users can search cases by title/description
• Search is debounced on the frontend
• Empty search results show a helpful message
• Admin can also search cases
• Search works in combination with existing filters