Generate a new scene with AI-generated video plan.

This endpoint:

1. Creates a new scene at the end of the project
2. Optionally processes uploaded reference images
3. Generates a video plan using LLM (keyframes and motion videos)
4. Saves keyframe and video records to the database
5. Returns the newly created scene with its plan

Args: project_id: Project ID to add scene to description: Description of the scene to generate duration: Optional duration in milliseconds. If not provided (None/empty), uses 'auto' mode where duration is determined by ai_video_generation.py based on voiceover length files: Optional reference images for generation current_user: Authenticated user db: Database session

Returns: Scene response with generated plan