Add background thumbnail generator to media API

- Implemented a background worker for generating video thumbnails, enhancing media processing capabilities.
- Added startup and shutdown events to manage the thumbnail generator's lifecycle.
- Refactored thumbnail generation logic to handle file processing more robustly, including checks for existing thumbnails and file accessibility.
- Updated existing functions to integrate with the new background processing approach, ensuring seamless thumbnail generation on demand.

management-dashboard-web-app/src/components/MqttDebugPanel.tsx

@@ -118,3 +118,5 @@ export const MqttDebugPanel: React.FC<MqttDebugPanelProps> = ({ isOpen, onClose
     )
 }
 
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