feat: search analytics tracking

[arrrnav]

Track usage metrics to inform future improvements.

Metrics to collect

• Most-searched terms
• Zero-result query rate
• Average result count per query

Approach

• Log query metadata server-side on each /search call (query, result count, elapsed time)
• Storage options: append to a local JSONL file, or a lightweight DB (SQLite)
• Optionally expose a /analytics endpoint or admin dashboard