Update Keyframe Project Level

Update/edit a project-level keyframe (video-centric, no scene_id required).

This is the video-centric version that looks up keyframes by project_id instead of scene_id. Used for keyframes that are not tied to a specific scene.

Args: preview: If True, generates image and returns URL without persisting to DB. commit_url: If provided, skips generation and commits this URL to DB directly.