refactor(learning): split answer idempotency registry

Tiku.Infrastructure/Persistence/ModulePersistence.cs

@@ -145,6 +145,7 @@ public interface ILearningPersistence : IModulePersistence
     DbSet<PracticeSessionQuestion> PracticeSessionQuestions { get; }
     DbSet<AnswerRecord> AnswerRecords { get; }
     DbSet<CurrentAnswer> CurrentAnswers { get; }
+    DbSet<AnswerSubmissionIdempotency> AnswerSubmissionIdempotencies { get; }
     DbSet<LearningOperationIdempotency> LearningOperationIdempotencies { get; }
     DbSet<LearningOutboxMessage> LearningOutboxMessages { get; }
     DbSet<LearningProjectionReceipt> LearningProjectionReceipts { get; }