feat(exam): save registration timestamp

models/exams

@@ -34,6 +34,7 @@ ExamRegistration
   exam ExamId
   user UserId
   occurrence ExamOccurrenceId Maybe
+  time UTCTime default=now()
   UniqueExamRegistration exam user
 ExamPartResult
   examPart ExamPartId

src/Handler/Exam.hs

@@ -815,7 +815,8 @@ postERegisterR tid ssh csh examn = do
   formResult btnResult $ \case
     BtnRegister -> do
       runDB $ do
-        insert_ $ ExamRegistration eId uid Nothing
+        now <- liftIO getCurrentTime
+        insert_ $ ExamRegistration eId uid Nothing now
         audit' $ TransactionExamRegister (unTermKey tid) (unSchoolKey ssh) csh examn userIdent
       addMessageI Success $ MsgExamRegisteredSuccess examn
       redirect $ CExamR tid ssh csh examn EShowR