Add isolated question completion credits

api/QuestionsApi/Models/CreditModels.cs

@@ -0,0 +1,35 @@
+using MongoDB.Bson;
+using MongoDB.Bson.Serialization.Attributes;
+
+namespace QuestionsApi.Models;
+
+public static class CreditEconomy
+{
+    public const int RollingEarningLimit = 20;
+    public static readonly TimeSpan EarningWindow = TimeSpan.FromHours(24);
+}
+
+public sealed class CreditEvent
+{
+    public string OperationId { get; set; } = "";
+    public DateTime CreatedUtc { get; set; }
+}
+
+public sealed class UserCreditAccount
+{
+    [BsonId]
+    [BsonRepresentation(BsonType.ObjectId)]
+    public string? Id { get; set; }
+    public string UserId { get; set; } = "";
+    public int Credits { get; set; }
+    public List<CreditEvent> RecentEarnings { get; set; } = [];
+    public DateTime UpdatedUtc { get; set; } = DateTime.UtcNow;
+}
+
+public sealed class CreditStatusResponse
+{
+    public int Credits { get; set; }
+    public int EarnedInRollingWindow { get; set; }
+    public int RemainingEarningCapacity { get; set; }
+    public int RollingWindowHours { get; set; } = 24;
+}

api/QuestionsApi/Models/QuestionResponse.cs

@@ -141,6 +141,7 @@ public class FirstQuestionAttemptResponse
     public Dictionary<string, int> ChoiceSelectionPercentages { get; set; } = [];
     public int QuestionElo { get; set; } = 1000;
     public RatingChangeResponse? Rating { get; set; }
+    public CreditStatusResponse? Credits { get; set; }
 }
 
 public class SkipQuestionResponse