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

src/openrouter_monitor/config.py

@@ -56,6 +56,10 @@ class Settings(BaseSettings):
         default=60,
         description="Background sync interval in minutes"
     )
+    usage_stats_retention_days: int = Field(
+        default=365,
+        description="Retention period for usage stats in days"
+    )
     
     # Limits
     max_api_keys_per_user: int = Field(