Get or create a folder using the get-or-create pattern (idempotent).

This endpoint checks if a folder exists at the given normalized_path. If it exists, returns the existing folder_id. If not, creates a new folder and returns the new folder_id.

Authorization: Any user belonging to an organization

Request: normalized_path: Relative path (e.g., "avena-fatua" or "mycology/avena-fatua") description: Optional description for the folder (defaults to empty string)

Response: folder_id: UUID of the existing or newly created folder