fix(cache): enhance sequence generation atomicity and concurrency safety

Implement Lua scripts for atomic sequence incrementing and cold-start initialization in Redis to prevent race conditions. Improve the sequence buffer manager with CAS-like logic to prevent interval leakage during concurrent allocations. Update the message repository to use `SELECT ... FOR UPDATE` and conditional updates for `last_seq` to ensure database consistency during high concurrency.

- Add `nextSeqLua` and `initSeqLua` for atomic Redis operations
- Implement thread-safe buffer management in `seq_buffer.go`
- Add row-level locking in `message_repo.go`
- Update service layers to support group-aware sequence retrieval

internal/service/group_service.go

@@ -473,7 +473,7 @@ func (s *groupService) broadcastMemberJoinNotice(groupID string, targetUserID st
 			}
 			// 从 Redis 获取下一个 seq
 			if s.conversationCache != nil {
-				seq, seqErr := s.conversationCache.GetNextSeq(context.Background(), conv.ID)
+				seq, seqErr := s.conversationCache.GetNextSeqWithGroup(context.Background(), conv.ID, true)
 				if seqErr != nil {
 					zap.L().Warn("get next seq failed, skipping system message",
 						zap.String("convID", conv.ID),
@@ -1398,7 +1398,7 @@ func (s *groupService) MuteMember(userID string, groupID string, targetUserID st
 
 			// 从 Redis 获取下一个 seq
 			if s.conversationCache != nil {
-				seq, seqErr := s.conversationCache.GetNextSeq(context.Background(), conv.ID)
+				seq, seqErr := s.conversationCache.GetNextSeqWithGroup(context.Background(), conv.ID, true)
 				if seqErr != nil {
 					zap.L().Warn("get next seq failed, skipping system message",
 						zap.String("convID", conv.ID),