feat(services): T31 implement statistics aggregation service

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

export/progress.md

@@ -88,12 +88,15 @@
 **Test totali API keys:** 38 test (25 router + 13 schema)
 **Coverage router:** 100%
 
-### 📊 Dashboard & Statistiche (T30-T34) - 1/5 completati
+### 📊 Dashboard & Statistiche (T30-T34) - 2/5 completati
 - [x] T30: Creare Pydantic schemas per stats - ✅ Completato (2026-04-07 17:45)
   - Creato: UsageStatsCreate, UsageStatsResponse, StatsSummary, StatsByModel, StatsByDate, DashboardResponse
   - Test: 16 test passanti, 100% coverage su schemas/stats.py
-- [ ] T31: Implementare servizio aggregazione stats 🟡 In progress
-- [ ] T32: Implementare endpoint GET /api/stats
+- [x] T31: Implementare servizio aggregazione stats - ✅ Completato (2026-04-07 18:30)
+  - Creato: get_summary(), get_by_model(), get_by_date(), get_dashboard_data()
+  - Query SQLAlchemy con join ApiKey per filtro user_id
+  - Test: 11 test passanti, 84% coverage su services/stats.py
+- [ ] T32: Implementare endpoint GET /api/stats/dashboard 🟡 In progress
 - [ ] T33: Implementare endpoint GET /api/usage
 - [ ] T34: Scrivere test per stats endpoints