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/message_service.go

@@ -90,7 +90,7 @@ func (s *MessageService) SendMessage(ctx context.Context, senderID, receiverID s
 
 	// 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, false)
 		if seqErr != nil {
 			return nil, fmt.Errorf("failed to get next seq for conversation %s: %w", conv.ID, seqErr)
 		}