Refactor codebase to follow best practices

[google-labs-jules]

This commit refactors the entire codebase to follow best practices.

The key changes include:

• Configuration Management: Replaced lazy_static and hardcoded TOML file parsing with a dedicated config module, which reads settings from a settings.toml file and environment variables.
• Error Handling: Introduced a custom Error enum and Result type alias in a new error.rs module. This centralizes error handling and replaces panic! calls and simple string-based error responses with a more robust system.
• Database Abstraction: Created a db.rs module with a Db struct that encapsulates the DynamoDB client and its operations. This removes direct database logic from the request handlers.
• Dependency Updates: Updated numerous dependencies in Cargo.lock and added new dependencies like bcrypt and rusty_paseto.
• Handler Refactoring: Rewrote the register_user and verification handlers in auth.rs to use the new config, database, and error handling abstractions.
• Code Cleanup: Removed the now-unused eval_constants.rs file and cleaned up obj.rs by removing the lazy_static TOML parsing.

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PR created automatically by Jules for task 10418785591186511306

[google-labs-jules]

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