Add server-side ELO ratings

api/QuestionsApi/Services/QuestionAttemptStore.cs

@@ -26,7 +26,7 @@ public class QuestionAttemptStore
         _col.Indexes.CreateOne(new CreateIndexModel<QuestionAttempt>(questionIndex));
     }
 
-    public async Task RecordFirstAttemptAsync(string questionExternalId, string userId, string choiceId, bool isCorrect)
+    public async Task<bool> RecordFirstAttemptAsync(string questionExternalId, string userId, string choiceId, bool isCorrect, bool isSkip = false)
     {
         var attempt = new QuestionAttempt
         {
@@ -34,15 +34,18 @@ public class QuestionAttemptStore
             UserId = userId,
             ChoiceId = choiceId,
             IsCorrect = isCorrect,
+            IsSkip = isSkip,
             CreatedUtc = DateTime.UtcNow
         };
 
         try
         {
             await _col.InsertOneAsync(attempt);
+            return true;
         }
         catch (MongoWriteException ex) when (ex.WriteError.Category == ServerErrorCategory.DuplicateKey)
         {
+            return false;
         }
     }
 
@@ -58,7 +61,7 @@ public class QuestionAttemptStore
         if (ids.Count == 0)
             return [];
 
-        var attempts = await _col.Find(a => ids.Contains(a.QuestionExternalId)).ToListAsync();
+        var attempts = await _col.Find(a => ids.Contains(a.QuestionExternalId) && !a.IsSkip).ToListAsync();
         var output = new Dictionary<string, Dictionary<string, int>>();
 
         foreach (var group in attempts.GroupBy(a => a.QuestionExternalId))