用户服务 4 个 crate 合并为单一 user-service，按 DDD 限界上下文聚合

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/services/user-service/src/api.rs

@@ -0,0 +1,18 @@
+// 注册/业务类接口的统一请求/响应包装格式
+// 与 backend/CLAUDE.md 中的 API 公共约定保持一致
+
+use serde::{Deserialize, Serialize};
+
+#[derive(Deserialize)]
+pub struct ApiRequest<T> {
+    pub device: i32,
+    pub language: i32,
+    pub data: T,
+}
+
+#[derive(Serialize)]
+pub struct ApiResponse<T> {
+    pub success: bool,
+    pub message: String,
+    pub data: Option<T>,
+}