Add post-answer review and per-question loading

api/QuestionsApi/Models/QuestionRequests.cs

@@ -9,6 +9,7 @@ public class QuestionSearchRequest
     public int? DifficultyMax { get; set; }
     public int Limit { get; set; } = 40;
     public bool SupportsChoicePools { get; set; }
+    public List<string> ExcludeExternalIds { get; set; } = [];
 }
 
 public class UpsertQuestionRequest

api/QuestionsApi/Services/QuestionStore.cs

@@ -37,11 +37,20 @@ public class QuestionStore
             filters.Add(builder.Gte(q => q.Difficulty, req.DifficultyMin.Value));
         if (req.DifficultyMax is not null)
             filters.Add(builder.Lte(q => q.Difficulty, req.DifficultyMax.Value));
+        var excludedIds = req.ExcludeExternalIds
+            .Where(id => !string.IsNullOrWhiteSpace(id))
+            .Select(id => id.Trim())
+            .Distinct(StringComparer.Ordinal)
+            .ToList();
+        if (excludedIds.Count > 0)
+            filters.Add(builder.Nin(q => q.ExternalId, excludedIds));
 
         var filter = filters.Count == 0 ? builder.Empty : builder.And(filters);
         var limit = Math.Clamp(req.Limit, 1, 80);
-        var questions = await _col.Find(filter).Limit(limit).ToListAsync();
-        return questions.OrderBy(_ => Guid.NewGuid()).ToList();
+        return await _col.Aggregate()
+            .Match(filter)
+            .Sample(limit)
+            .ToListAsync();
     }
 
     public async Task<Question?> GetByIdAsync(string id) =>