Clears all in-memory data for this folder to free memory.
This method: - Clears the hierarchical tree structure (children) - Clears the flat file list (project_files) - Clears the in-memory tree structure
After calling this method, the folder will appear as if it has never been loaded. The next call to load_files_from_db() will reload all data from the database (since needs_reload() will return true).
This is useful for memory management when switching between projects or when you want to force a reload on the next access.
This does NOT update the database - it only clears in-memory state.