Create Audiobook

Create a new audiobook.

Accepts:

• user_id: User creating the audiobook
• title: Audiobook title (optional - auto-generated if empty)
• description: Story description
• character_ids: Character IDs as comma-separated string (e.g., "id1,id2,id3") - preferred
• character_id: Single character ID (for backward compatibility)
• page_count: Story length in pages (allowed: 8, 25, 50; default: 8)
• language: Language code (default: "en")
• is_public: Whether audiobook is public (default: False)
• animation_style: Illustration style id — cgi (default), 2d, anime, felt

Returns audiobook_id immediately and processes in background.

If title is empty or None, auto-generates a child-friendly title using AI. Note: If multiple character_ids provided, uses the first one for title generation and character context.