- Add GET /api/admin/stats/overview (user counts, login stats)
- Add GET /api/admin/stats/active-users (time series with period toggle)
- Add GET /api/admin/stats/actions (action breakdown with date filter)
- Add GET /api/admin/stats/peak-times (hourly/daily averages)
- Implement 5-minute in-memory caching for expensive aggregations
- All 57 Phase 3 tests passing