Backup and restore lists via JSON or CSV#937

It would be very useful to have a backup and restore feature for user data (lists, backlog, ratings, etc.) using formats like JSON or CSV.

This would allow users to:

• Back up their data locally
• Restore their data if needed
• Transfer their lists between accounts
• Edit data externally (especially with CSV)

Suggested functionality:

• Export all user data as JSON and/or CSV
• Import data from JSON/CSV with validation
• Option to merge or overwrite existing data
• Ability to restore specific lists instead of everything at once

This would improve data safety, portability, and give users more control over their collections.