feat(stats): added Providers dashboard with per-provider analytics and utilization insights
- Added `/api/stats/providers` endpoint and database queries returning per-provider request/token/cost totals, hourly burn, and time-series data.
- Created ProvidersRoute UI component with totals panel, trend charts, peak-hours histogram, and window-insights visualizations.
- Added usage-window analytics computing peak utilization fractions, ideal account counts, and exhaustion events from usage snapshots.
- Included comprehensive tests for the aggregation logic, API, and window-stat computations.
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