Backend Refactors & Advanced Error Handling

[7086cmd]

Major backend code improvements for reliability, errors, and maintainability.

Tasks:

• Implement thiserror throughout for errors
• Centralize ID types (typed ID wrappers)
• Migrate away from manual serialization to postcard where viable
• Replace custom TOTP with an audited crate
• Remove .unwrap() panics and improve error messaging
• Integrate geo for geometry, reduce code duplication
• Refactor challenge/response auth centrally

See refactoring plan for code spawn locations and details.