Perform SAM-3 magic select segmentation on a project-level keyframe using a bounding box.

This is the video-centric version that doesn't require scene_id. Keyframes are looked up by project_id instead of scene_id.

Args: project_id: Project ID keyframe_id: Keyframe ID box_x1-box_y2: Normalized coordinates (0-1) for selection box image_url: Optional override image URL to segment image: Optional image file to segment (used when the client has a blob URL that cannot be fetched by the server)

Returns: { success: bool, maskedImageUrl: str - URL of masked object (transparent background), maskOverlayUrl: str - URL of colored mask overlay for preview, bounds: { x, y, width, height } - Bounding box in pixels, imageWidth: int - Original image width, imageHeight: int - Original image height, }