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/scheduler.py

@@ -0,0 +1,76 @@
+"""APScheduler task scheduler.
+
+T55: Background task scheduler using APScheduler with AsyncIOScheduler.
+"""
+from apscheduler.schedulers.asyncio import AsyncIOScheduler
+
+
+# Singleton scheduler instance
+_scheduler = None
+
+
+def get_scheduler():
+    """Get or create the singleton scheduler instance.
+    
+    Returns:
+        AsyncIOScheduler: The scheduler instance (singleton)
+        
+    Example:
+        >>> scheduler = get_scheduler()
+        >>> scheduler.start()
+    """
+    global _scheduler
+    if _scheduler is None:
+        _scheduler = AsyncIOScheduler(timezone='UTC')
+    return _scheduler
+
+
+def scheduled_job(trigger, **trigger_args):
+    """Decorator to register a scheduled job.
+    
+    Args:
+        trigger: APScheduler trigger (IntervalTrigger, CronTrigger, etc.)
+        **trigger_args: Additional arguments for add_job (id, name, etc.)
+        
+    Returns:
+        Decorator function that registers the job and returns original function
+        
+    Example:
+        >>> from apscheduler.triggers.interval import IntervalTrigger
+        >>> 
+        >>> @scheduled_job(IntervalTrigger(hours=1), id='sync_task')
+        ... async def sync_data():
+        ...     pass
+    """
+    def decorator(func):
+        get_scheduler().add_job(func, trigger=trigger, **trigger_args)
+        return func
+    return decorator
+
+
+def init_scheduler():
+    """Initialize and start the scheduler.
+    
+    Should be called during application startup.
+    Registers all decorated jobs and starts the scheduler.
+    
+    Example:
+        >>> init_scheduler()
+        >>> # Scheduler is now running
+    """
+    scheduler = get_scheduler()
+    scheduler.start()
+
+
+def shutdown_scheduler():
+    """Shutdown the scheduler gracefully.
+    
+    Should be called during application shutdown.
+    Waits for running jobs to complete before stopping.
+    
+    Example:
+        >>> shutdown_scheduler()
+        >>> # Scheduler is stopped
+    """
+    scheduler = get_scheduler()
+    scheduler.shutdown(wait=True)