feat: scaffold Rust/Axum backend

backend/src/error.rs

@@ -0,0 +1,30 @@
+use axum::{http::StatusCode, response::{IntoResponse, Response}, Json};
+use serde_json::json;
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum AppError {
+    #[error("database error: {0}")]
+    Db(#[from] sqlx::Error),
+    #[error("not found")]
+    NotFound,
+    #[error("conflict: {0}")]
+    Conflict(String),
+    #[error("unauthorized")]
+    Unauthorized,
+    #[error("bad request: {0}")]
+    BadRequest(String),
+}
+
+impl IntoResponse for AppError {
+    fn into_response(self) -> Response {
+        let (status, msg) = match &self {
+            AppError::Db(_) => (StatusCode::INTERNAL_SERVER_ERROR, "internal error".into()),
+            AppError::NotFound => (StatusCode::NOT_FOUND, "not found".into()),
+            AppError::Conflict(m) => (StatusCode::CONFLICT, m.clone()),
+            AppError::Unauthorized => (StatusCode::UNAUTHORIZED, "unauthorized".into()),
+            AppError::BadRequest(m) => (StatusCode::BAD_REQUEST, m.clone()),
+        };
+        (status, Json(json!({"error": msg}))).into_response()
+    }
+}