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

tests/unit/tasks/test_cleanup.py

@@ -0,0 +1,107 @@
+"""Tests for cleanup tasks.
+
+T58: Task to clean up old usage stats data.
+"""
+import pytest
+from datetime import datetime, date, timedelta
+from unittest.mock import Mock, patch, MagicMock, AsyncMock
+from apscheduler.triggers.cron import CronTrigger
+
+
+@pytest.mark.unit
+class TestCleanupOldUsageStats:
+    """Test suite for cleanup_old_usage_stats task."""
+    
+    def test_cleanup_has_correct_decorator(self):
+        """Test that cleanup_old_usage_stats has correct scheduled_job decorator."""
+        # Arrange
+        from openrouter_monitor.tasks.cleanup import cleanup_old_usage_stats
+        from openrouter_monitor.tasks.scheduler import get_scheduler
+        
+        # Act
+        scheduler = get_scheduler()
+        job = scheduler.get_job('cleanup_old_usage_stats')
+        
+        # Assert
+        assert job is not None
+        assert job.func == cleanup_old_usage_stats
+        assert isinstance(job.trigger, CronTrigger)
+    
+    def test_cleanup_is_async_function(self):
+        """Test that cleanup_old_usage_stats is an async function."""
+        # Arrange
+        from openrouter_monitor.tasks.cleanup import cleanup_old_usage_stats
+        import inspect
+        
+        # Assert
+        assert inspect.iscoroutinefunction(cleanup_old_usage_stats)
+    
+    @pytest.mark.asyncio
+    async def test_cleanup_handles_errors_gracefully(self):
+        """Test that cleanup handles errors without crashing."""
+        # Arrange
+        from openrouter_monitor.tasks.cleanup import cleanup_old_usage_stats
+        
+        with patch('openrouter_monitor.tasks.cleanup.SessionLocal') as mock_session:
+            # Simulate database error
+            mock_session.side_effect = Exception("Database connection failed")
+            
+            # Act & Assert - should not raise
+            await cleanup_old_usage_stats()
+    
+    @pytest.mark.asyncio
+    async def test_cleanup_uses_retention_days_from_config(self):
+        """Test that cleanup uses retention days from settings."""
+        # Arrange
+        from openrouter_monitor.tasks.cleanup import cleanup_old_usage_stats
+        from openrouter_monitor.config import get_settings
+        
+        mock_result = MagicMock()
+        mock_result.rowcount = 0
+        
+        async def mock_execute(*args, **kwargs):
+            return mock_result
+        
+        mock_db = MagicMock()
+        mock_db.execute = mock_execute
+        mock_db.commit = Mock()
+        
+        # Get actual retention days from config
+        settings = get_settings()
+        expected_retention = settings.usage_stats_retention_days
+        
+        with patch('openrouter_monitor.tasks.cleanup.SessionLocal') as mock_session:
+            mock_session.return_value.__enter__ = Mock(return_value=mock_db)
+            mock_session.return_value.__exit__ = Mock(return_value=False)
+            
+            # Act
+            await cleanup_old_usage_stats()
+            
+            # Assert - verify retention days is reasonable (default 365)
+            assert expected_retention > 0
+            assert expected_retention <= 365 * 5  # Max 5 years
+
+
+@pytest.mark.unit
+class TestCleanupConfiguration:
+    """Test suite for cleanup configuration."""
+    
+    def test_retention_days_configurable(self):
+        """Test that retention days is configurable."""
+        from openrouter_monitor.config import get_settings
+        
+        settings = get_settings()
+        
+        # Should have a default value
+        assert hasattr(settings, 'usage_stats_retention_days')
+        assert isinstance(settings.usage_stats_retention_days, int)
+        assert settings.usage_stats_retention_days > 0
+    
+    def test_default_retention_is_one_year(self):
+        """Test that default retention period is approximately one year."""
+        from openrouter_monitor.config import get_settings
+        
+        settings = get_settings()
+        
+        # Default should be 365 days (1 year)
+        assert settings.usage_stats_retention_days == 365