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
- Add generate_api_token with format 'or_api_' + 48 bytes random
- Implement hash_token with SHA-256
- Add verify_api_token with timing-safe comparison (secrets.compare_digest)
- Only hash stored in DB, plaintext shown once
- 20 comprehensive tests with 100% coverage
- Handle TypeError for non-string inputs
