Add statistics router with two endpoints:
- GET /api/stats/dashboard: Aggregated dashboard statistics
- Query param: days (1-365, default 30)
- Auth required
- Returns DashboardResponse
- GET /api/usage: Detailed usage statistics with filtering
- Required params: start_date, end_date
- Optional filters: api_key_id, model
- Pagination: skip, limit (max 1000)
- Auth required
- Returns List[UsageStatsResponse]
Also add get_usage_stats() service function for querying
individual usage records with filtering and pagination.
