Add balance handling to card transactions and user accounts

Signed-off-by: Dmitrii <computer@yawaflua.tech>

Took 6 minutes

backend/SpMega.Backend/Persistent/Models/Users/Card.cs

@@ -9,6 +9,8 @@ public class Card
     public string Name { get; set; }
     public string SpworldsID { get; set; }
     public string Token { get; set; }
+
+    public int Balance { get; set; } = 0;
     
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
     public DateTime UpdatedAt { get; set; }