fix: DynamoDBのイベント取得順をpayload.timestampに統一

server/internal/repository/dynamodb/event.go

@@ -135,14 +135,28 @@ func (r *EventRepository) FindBySessionID(ctx context.Context, sessionID string)
 		events[i] = r.itemToEvent(&item)
 	}
 
-	// Sort by created_at since sort_key is now uuid-based and doesn't preserve chronological order
-	sort.Slice(events, func(i, j int) bool {
-		return events[i].CreatedAt.Before(events[j].CreatedAt)
+	// sort_key is uuid-based, so chronological order must be reconstructed here.
+	// Match the other backends: sort by payload.timestamp, not created_at.
+	sort.SliceStable(events, func(i, j int) bool {
+		return getTimestampFromPayload(events[i]).Before(getTimestampFromPayload(events[j]))
 	})
 
 	return events, nil
 }
 
+func getTimestampFromPayload(e *domain.Event) time.Time {
+	if ts, ok := e.Payload["timestamp"].(string); ok {
+		if parsed, err := time.Parse(time.RFC3339Nano, ts); err == nil {
+			return parsed
+		}
+		// Try parsing without timezone
+		if parsed, err := time.Parse("2006-01-02T15:04:05.000Z", ts); err == nil {
+			return parsed
+		}
+	}
+	return e.CreatedAt
+}
+
 func (r *EventRepository) CountBySessionID(ctx context.Context, sessionID string) (int, error) {
 	keyCond := expression.Key("session_id").Equal(expression.Value(sessionID))
 

server/internal/repository/testsuite/event_suite.go

@@ -228,6 +228,55 @@ func (s *EventRepositorySuite) TestFindBySessionID_ChronologicalOrder() {
 	s.WithinDuration(baseTime.Add(500*time.Millisecond), events[4].CreatedAt, time.Microsecond, "Last event should have latest timestamp")
 }
 
+// TestFindBySessionID_OrdersByPayloadTimestamp ensures events are returned ordered by
+// payload.timestamp (event time), not created_at — guarding the cross-backend contract.
+func (s *EventRepositorySuite) TestFindBySessionID_OrdersByPayloadTimestamp() {
+	ctx := context.Background()
+
+	sessionID := s.createTestSession("event-payload-ts-order")
+	if sessionID == "" {
+		s.T().Skip("SessionRepo not available, skipping test")
+	}
+
+	// Second precision + UTC "Z" keeps the timestamp string lexically ordered,
+	// which Postgres relies on (ORDER BY payload->>'timestamp'), while Go-based
+	// backends parse it with RFC3339Nano. Fixed width avoids fractional-trim pitfalls.
+	baseTime := time.Now().UTC().Truncate(time.Second)
+
+	const n = 5
+	expectedTimestamps := make([]string, n)
+	for k := 0; k < n; k++ {
+		// payload.timestamp ascends with k; created_at descends with k (reverse order).
+		ts := baseTime.Add(time.Duration(k) * time.Second).Format(time.RFC3339Nano)
+		expectedTimestamps[k] = ts
+		event := &domain.Event{
+			SessionID: sessionID,
+			UUID:      uuid.New().String(),
+			EventType: "message",
+			Payload: map[string]interface{}{
+				"timestamp": ts,
+				"marker":    k,
+			},
+			CreatedAt: baseTime.Add(time.Duration(n-k) * time.Hour),
+		}
+		err := s.Repo.Create(ctx, event)
+		s.Require().NoError(err)
+	}
+
+	events, err := s.Repo.FindBySessionID(ctx, sessionID)
+	s.Require().NoError(err)
+	s.Require().Len(events, n)
+
+	// Returned order must follow payload.timestamp ascending, i.e. the insertion (created_at)
+	// order reversed. If a backend sorted by created_at, this would come back descending and fail.
+	for i := 0; i < n; i++ {
+		ts, ok := events[i].Payload["timestamp"].(string)
+		s.Require().True(ok, "event[%d] should carry a string payload.timestamp", i)
+		s.Equal(expectedTimestamps[i], ts,
+			"events should be ordered by payload.timestamp ascending (position %d)", i)
+	}
+}
+
 func (s *EventRepositorySuite) TestFindBySessionID_Empty() {
 	ctx := context.Background()