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/group_service.go

@@ -474,13 +474,13 @@ func (s *groupService) broadcastMemberJoinNotice(groupID string, targetUserID st
 			if s.conversationCache != nil {
 				seq, seqErr := s.conversationCache.GetNextSeq(context.Background(), conv.ID)
 				if seqErr != nil {
-					zap.L().Warn("redis get next seq failed, falling back to DB",
+					zap.L().Warn("get next seq failed, skipping system message",
 						zap.String("convID", conv.ID),
 						zap.Error(seqErr),
 					)
-				} else {
-					msg.Seq = seq
+					return
 				}
+				msg.Seq = seq
 			}
 			if err := s.messageRepo.CreateMessageWithSeq(msg); err != nil {
 				zap.L().Warn("保存入群提示消息失败",
@@ -1399,13 +1399,13 @@ func (s *groupService) MuteMember(userID string, groupID string, targetUserID st
 			if s.conversationCache != nil {
 				seq, seqErr := s.conversationCache.GetNextSeq(context.Background(), conv.ID)
 				if seqErr != nil {
-					zap.L().Warn("redis get next seq failed, falling back to DB",
+					zap.L().Warn("get next seq failed, skipping system message",
 						zap.String("convID", conv.ID),
 						zap.Error(seqErr),
 					)
-				} else {
-					msg.Seq = seq
+					return nil
 				}
+				msg.Seq = seq
 			}
 
 			if err := s.messageRepo.CreateMessageWithSeq(msg); err != nil {