Expose AI usage analytics data endpoints for tracking API consumption, costs, and popularity metrics, including total requests, successful/failed, avg response time, popular destinations, and estimated OpenAI API cost. Also, per-user usage analytics.
## Description Expose AI usage analytics data endpoints for tracking API consumption, costs, and popularity metrics. ## Acceptance Criteria - [ ] `GET /api/ai/analytics/usage?period=month` returns: total requests, successful/failed, avg response time - [ ] `GET /api/ai/analytics/popular-destinations` returns top 10 most-requested destinations - [ ] `GET /api/ai/analytics/cost-estimate` returns estimated OpenAI API cost for period - [ ] Per-user usage: `GET /api/ai/analytics/usage/me` — personal AI usage stats - [ ] Usage data aggregated from `ai_schema.ai_request_logs` table (#4) - [ ] Caching: analytics endpoints cached 5 minutes (not real-time critical) - [ ] Admin-only: global analytics; users see only their own stats - [ ] Response includes comparison to previous period (trend arrows) - [ ] Unit tests for aggregation queries ## Technical Notes - Depends on: #4 (request logging), #39 (AI setup) - Use JPA `@Query` with GROUP BY for aggregations - Consider materialized view or sche