fix(auth): 修复新用户注册Uid超长导致截断错误

- GenerateUid生成的是timestamp+random(14位)，超过数据库nvarchar(6)限制
- 改为生成6位随机数字，并查库确保唯一性

server/MiAssessment/src/MiAssessment.Core/Services/UserService.cs

@@ -59,8 +59,8 @@ public class UserService : BaseService<User, int>, IUserService
         if (dto == null)
             throw new ArgumentNullException(nameof(dto));
 
-        // Generate UID
-        var uid = GenerateUid();
+        // 生成唯一UID
+        var uid = await GenerateUidAsync();
 
         var user = new User
         {
@@ -155,14 +155,20 @@ public class UserService : BaseService<User, int>, IUserService
     }
 
     /// <summary>
-    /// Generate a unique UID for new user
+    /// 生成6位唯一用户UID
     /// </summary>
-    private string GenerateUid()
+    private async Task<string> GenerateUidAsync()
     {
-        // Generate a numeric UID based on timestamp and random number
-        var timestamp = DateTimeOffset.UtcNow.ToUnixTimeSeconds();
-        var random = Random.Shared.Next(1000, 9999);
-        return $"{timestamp}{random}";
+        const int maxAttempts = 10;
+        for (var i = 0; i < maxAttempts; i++)
+        {
+            var uid = Random.Shared.Next(100000, 999999).ToString();
+            var exists = await _dbSet.AnyAsync(u => u.Uid == uid);
+            if (!exists)
+                return uid;
+        }
+        // 兜底：使用时间戳后6位
+        return DateTimeOffset.UtcNow.ToUnixTimeMilliseconds().ToString()[^6..];
     }
 
     /// <summary>