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/tasks/cleanup.py

@@ -0,0 +1,59 @@
+"""Cleanup tasks for old data.
+
+T58: Task to clean up old usage stats data.
+"""
+import logging
+from datetime import datetime, timedelta
+
+from apscheduler.triggers.cron import CronTrigger
+from sqlalchemy import delete
+
+from openrouter_monitor.database import SessionLocal
+from openrouter_monitor.models.usage_stats import UsageStats
+from openrouter_monitor.config import get_settings
+from openrouter_monitor.tasks.scheduler import scheduled_job
+
+
+logger = logging.getLogger(__name__)
+settings = get_settings()
+
+
+@scheduled_job(
+    CronTrigger(day_of_week='sun', hour=3, minute=0),
+    id='cleanup_old_usage_stats',
+    replace_existing=True
+)
+async def cleanup_old_usage_stats():
+    """Clean up usage stats older than retention period.
+    
+    Runs weekly on Sunday at 3:00 AM UTC.
+    Removes UsageStats records older than usage_stats_retention_days
+    (default: 365 days).
+    
+    The retention period is configurable via the
+    USAGE_STATS_RETENTION_DAYS environment variable.
+    """
+    logger.info("Starting cleanup of old usage stats")
+    
+    try:
+        with SessionLocal() as db:
+            # Calculate cutoff date
+            retention_days = settings.usage_stats_retention_days
+            cutoff_date = datetime.utcnow().date() - timedelta(days=retention_days)
+            
+            logger.info(f"Removing usage stats older than {cutoff_date}")
+            
+            # Delete old records
+            stmt = delete(UsageStats).where(UsageStats.date < cutoff_date)
+            result = db.execute(stmt)
+            deleted_count = result.rowcount
+            
+            db.commit()
+            
+            logger.info(
+                f"Cleanup completed. Deleted {deleted_count} old usage stats records "
+                f"(retention: {retention_days} days)"
+            )
+            
+    except Exception as e:
+        logger.error(f"Error in cleanup_old_usage_stats job: {e}")