fix: 修复数据库与SignalR无法连接的问题

server/src/Database/ExamManager.cs

@@ -8,12 +8,7 @@ public class ExamManager
 {
     private static NLog.Logger logger = NLog.LogManager.GetCurrentClassLogger();
 
-    private readonly AppDataConnection _db;
-
-    public ExamManager(AppDataConnection db)
-    {
-        this._db = db;
-    }
+    private AppDataConnection _db = new();
 
     /// <summary>
     /// 创建新实验

server/src/Database/ResourceManager.cs

@@ -9,12 +9,7 @@ public class ResourceManager
 {
     private static NLog.Logger logger = NLog.LogManager.GetCurrentClassLogger();
 
-    private readonly AppDataConnection _db;
-
-    public ResourceManager(AppDataConnection db)
-    {
-        this._db = db;
-    }
+    private readonly AppDataConnection _db = new();
 
     /// <summary>
     /// 根据文件扩展名获取MIME类型

server/src/Database/UserManager.cs

@@ -8,12 +8,7 @@ public class UserManager
 {
     private static NLog.Logger logger = NLog.LogManager.GetCurrentClassLogger();
 
-    private readonly AppDataConnection _db;
-
-    public UserManager(AppDataConnection db)
-    {
-        this._db = db;
-    }
+    private readonly AppDataConnection _db = new();
 
     /// <summary>
     /// 添加一个新的用户到数据库