A: Define custom error types (or use existing ones) and implement Responder for them, so your API can convert internal errors to HTTP responses (e.g. 400/404/500). Use Result<T, YourError> in route handlers and ? propagation. For database errors, map Diesel errors (e.g. NotFound, UniqueViolation) to appropriate HTTP status codes. Also, you may use JSON error envelope formats (e.g. { “error”: “message”, “code”: 400 }) for consistency.
