feat(tasks): T55-T58 implement background tasks for OpenRouter sync

- T55: Setup APScheduler with AsyncIOScheduler and @scheduled_job decorator
- T56: Implement hourly usage stats sync from OpenRouter API
- T57: Implement daily API key validation job
- T58: Implement weekly cleanup job for old usage stats
- Add usage_stats_retention_days config option
- Integrate scheduler with FastAPI lifespan events
- Add 26 unit tests for scheduler, sync, and cleanup tasks
- Add apscheduler to requirements.txt

The background tasks now automatically:
- Sync usage stats every hour from OpenRouter
- Validate API keys daily at 2 AM UTC
- Clean up old data weekly on Sunday at 3 AM UTC

export/progress.md

@@ -160,11 +160,29 @@
 - [ ] T53: Implementare router /keys
 - [ ] T54: Aggiungere HTMX per azioni CRUD
 
-### ⚙️ Background Tasks (T55-T58) - 0/4 completati
-- [ ] T55: Configurare APScheduler
-- [ ] T56: Implementare task sync usage stats
-- [ ] T57: Implementare task validazione key
-- [ ] T58: Integrare scheduler in startup app
+### ⚙️ Background Tasks (T55-T58) - 4/4 completati ✅
+- [x] T55: Configurare APScheduler - ✅ Completato (2026-04-07 20:30)
+  - Creato: AsyncIOScheduler singleton con timezone UTC
+  - Creato: Decorator @scheduled_job per registrare task
+  - Integrato: FastAPI lifespan per startup/shutdown
+  - Test: 10 test passanti
+- [x] T56: Implementare task sync usage stats - ✅ Completato (2026-04-07 20:30)
+  - Task: sync_usage_stats ogni ora (IntervalTrigger)
+  - Features: Decripta key, chiama OpenRouter /usage, upsert in UsageStats
+  - Rate limiting: 0.35s tra richieste (20 req/min)
+  - Date range: ultimi 7 giorni
+  - Test: 6 test passanti
+- [x] T57: Implementare task validazione key - ✅ Completato (2026-04-07 20:30)
+  - Task: validate_api_keys giornaliero alle 2:00 AM (CronTrigger)
+  - Features: Decripta key, chiama OpenRouter /auth/key, disattiva key invalide
+  - Test: 4 test passanti
+- [x] T58: Implementare task cleanup dati vecchi - ✅ Completato (2026-04-07 20:30)
+  - Task: cleanup_old_usage_stats settimanale domenica 3:00 AM
+  - Features: Rimuove UsageStats più vecchi di 365 giorni (configurabile)
+  - Test: 6 test passanti
+
+**Progresso sezione:** 100% (4/4 task)
+**Test totali tasks:** 26 test passanti
 
 ### 🔒 Sicurezza & Hardening (T59-T63) - 0/5 completati
 - [ ] T59: Implementare security headers middleware