Reference to FileHistory record where this file was deleted.
If delete_id = 0, the file is not deleted. If delete_id > 0, the file is deleted and delete_id points to file_history.id where change_type="deleted".
Deleted files are flagged rather than immediately removed from database, allowing for cleanup operations and UI updates. Files with delete_id > 0 should be excluded from UI lists and scanning operations.
Provides direct link to deletion information (timestamp, agent, etc.) via the FileHistory record.