Enhances chapter pagination

[FinnTheHero]

Updates chapter pagination to leverage a numerical index, which provides a more robust and efficient cursor mechanism.

• Simplifies cursor handling by transitioning from a string-based document ID to an integer index.
• Removes an external natural sorting dependency, streamlining the retrieval process.
• Optimizes data transfer by simplifying the frontend chapter structure to include only essential fields.
• Improves error handling by returning a 404 status when no chapters are found for a novel.