fix(backend): remove unnecessary dereference in backup operation

- Simplify Backup::new call by removing redundant dereference
- MutexGuard already implements DerefMut

src-tauri/src/database.rs

@@ -241,7 +241,7 @@ impl Database {
         // 使用 Backup 将临时库原子写回主库
         {
             let mut main_conn = lock_conn!(self.conn);
-            let backup = Backup::new(&temp_conn, &mut *main_conn)
+            let backup = Backup::new(&temp_conn, &mut main_conn)
                 .map_err(|e| AppError::Database(e.to_string()))?;
             backup
                 .step(-1)