feat: domain filtering for search results

[arrrnav]

Allow users to restrict results to a specific domain (e.g. ics.uci.edu, cs.uci.edu).

Approach

• Domain filtering can be applied post-retrieval — no index changes needed
• URLs are already stored in id_to_url.json; extract the domain and filter before returning results
• Expose as a query parameter on the /search endpoint: ?q=...&domain=ics.uci.edu

Frontend

• Add a domain filter input or dropdown on the results page