Refactor codebase for idiomatic Rust

[kacy]

Summary

• Comprehensive refactoring to make the codebase more idiomatic and cleaner
• Improved error handling with proper error types instead of panics
• More flexible APIs using impl Into<String> patterns
• Better documentation and test coverage

Changes

Error Handling

• Added LogoutError type - logout now returns Result instead of panicking on missing token
• Added DbError enum for database operations with Serialization and Request variants
• insert() and update() now return Result to handle serialization errors

Code Quality

• Removed unnecessary braces in imports (use reqwest::{Client} → use reqwest::Client)
• Removed redundant type annotations throughout
• Used #[derive(Clone)] instead of manual implementation for Claims
• Removed async from jwt_valid() (no await needed)
• Removed debug println! statements from production code
• Renamed Password struct to Credentials for clarity
• Used &str in request structs with serde for efficiency
• Added helper method add_filter() to reduce code duplication

API Improvements

• Added set_bearer_token() method for cleaner token setting
• Filter methods now accept impl Into<String> for flexibility
• in_() method now accepts any IntoIterator of string-like values

Modern Rust Patterns

• Used let-else patterns (let Some(x) = ... else { return })
• Used format string interpolation ({e} instead of {}, e)
• Improved doc comments with proper /// format

Tests

• Added test_client_creation with explicit values
• Added test_set_bearer_token
• Added test_logout_requires_bearer_token
• Added test_db_error_display
• Increased test count from 15 to 19