Add statistics aggregation service with 4 core functions:
- get_summary(): Aggregates total requests, cost, tokens with avg cost
- get_by_model(): Groups stats by model with percentage calculations
- get_by_date(): Groups stats by date for time series data
- get_dashboard_data(): Combines all stats for dashboard view
Features:
- SQLAlchemy queries with ApiKey join for user filtering
- Decimal precision for all monetary values
- Period calculation and percentage breakdowns
- Top models extraction
Test: 11 unit tests covering all aggregation functions
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