Update an existing asset with a new generated image. This is used for asset revisions - instead of creating a new asset, the existing asset's image_url is updated.

Args: project_id: Project ID containing the asset asset_id: Asset ID to update prompt: New text description of the asset files: Optional list of reference images for generation preview: If True, generates image and returns URL without persisting to DB. User can then accept (call again with preview=False) or discard. commit_url: If provided, skips generation and commits this URL to DB directly. Used when accepting a previewed image. current_user: Authenticated user db: Database session

Returns: Updated asset data with new image URL