feat(cache): implement database fallback for conversation sequence management

Improve the reliability of conversation sequence generation by implementing a fallback mechanism to the database when Redis is unavailable or when keys are missing.

- Add `GetNextSeq` to `MessageRepository` and implement it in `MessageRepositoryAdapter`.
- Update `ConversationCache` to fallback to DB in `GetConvMaxSeq`, `GetConvMaxSeqBatch`, and `GetNextSeq`.
- Ensure Redis TTL is refreshed after every `INCR` operation to prevent sequence counter expiration.
- Refactor service layers to handle sequence generation errors more explicitly.
- Add `IsEnabled` helper to `LayeredCache` for cleaner availability checks.

internal/service/chat_service.go

@@ -348,13 +348,9 @@ func (s *chatServiceImpl) SendMessage(ctx context.Context, senderID string, conv
 	if s.conversationCache != nil {
 		seq, err := s.conversationCache.GetNextSeq(ctx, conversationID)
 		if err != nil {
-			zap.L().Warn("redis get next seq failed, falling back to DB",
-				zap.String("convID", conversationID),
-				zap.Error(err),
-			)
-		} else {
-			message.Seq = seq
+			return nil, fmt.Errorf("failed to get next seq for conversation %s: %w", conversationID, err)
 		}
+		message.Seq = seq
 	}
 
 	// 使用事务创建消息并更新seq
@@ -916,13 +912,9 @@ func (s *chatServiceImpl) SaveMessage(ctx context.Context, senderID string, conv
 	if s.conversationCache != nil {
 		seq, err := s.conversationCache.GetNextSeq(ctx, conversationID)
 		if err != nil {
-			zap.L().Warn("redis get next seq failed, falling back to DB",
-				zap.String("convID", conversationID),
-				zap.Error(err),
-			)
-		} else {
-			message.Seq = seq
+			return nil, fmt.Errorf("failed to get next seq for conversation %s: %w", conversationID, err)
 		}
+		message.Seq = seq
 	}
 
 	if err := s.repo.CreateMessageWithSeq(message); err != nil {