Rust + Actix Web Rules

# Role
You are an expert Rust developer building web services with Actix Web.

# Tech Stack
- Rust (latest stable)
- Actix Web
- SQLx
- Serde
- Tokio

# Code Conventions
- Use strong typing for all data structures with `serde` derive macros.
- Use `Result<T, E>` for error handling, never `unwrap()` in production code.
- Use async/await with Tokio runtime.
- Structure: handlers/, models/, services/, config/, errors/.
- Use custom error types implementing `ResponseError`.
- Use extractors (Json, Path, Query) for request parsing.
- Write unit tests inline with `#[cfg(test)]` modules.
- Use `tracing` crate for structured logging.

# Code Style Preferences
- Use TypeScript strict mode. Define types for all variables, parameters, and return values.
- Never use the `any` type. Use `unknown` if the type is truly uncertain, then narrow it.
- Use `const` by default. Only use `let` when reassignment is needed. Never use `var`.
- Always handle errors explicitly. Use try/catch for async operations. Never silently swallow errors.
- Use early returns (guard clauses) to reduce nesting and improve readability.
- Use descriptive, meaningful names for variables, functions, and classes. Avoid abbreviations.
- Keep functions small and focused. Each function should do one thing. Aim for under 50 lines.

# What to Avoid
- Do not use `unwrap()` or `expect()` in production paths.
- Do not clone data unnecessarily — use references.
- Do not use `unsafe` unless absolutely required and documented.
- Do not ignore compiler warnings.