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/chat_service.go

@@ -481,7 +481,7 @@ func (s *chatServiceImpl) SendMessage(ctx context.Context, senderID string, conv
 
 	// 从 Redis 获取下一个 seq
 	if s.conversationCache != nil {
-		seq, err := s.conversationCache.GetNextSeq(ctx, conversationID)
+		seq, err := s.conversationCache.GetNextSeqWithGroup(ctx, conversationID, conv.Type == model.ConversationTypeGroup)
 		if err != nil {
 			return nil, fmt.Errorf("failed to get next seq for conversation %s: %w", conversationID, err)
 		}
@@ -1042,7 +1042,7 @@ func (s *chatServiceImpl) IsUserOnline(userID string) bool {
 // 适用于群聊等由调用方自行负责推送的场景
 func (s *chatServiceImpl) SaveMessage(ctx context.Context, senderID string, conversationID string, segments model.MessageSegments, replyToID *string) (*model.Message, error) {
 	// 验证会话是否存在
-	_, err := s.getConversation(ctx, conversationID)
+	conv, err := s.getConversation(ctx, conversationID)
 	if err != nil {
 		if errors.Is(err, gorm.ErrRecordNotFound) {
 			return nil, errors.New("会话不存在，请重新创建会话")
@@ -1075,7 +1075,7 @@ func (s *chatServiceImpl) SaveMessage(ctx context.Context, senderID string, conv
 
 	// 从 Redis 获取下一个 seq（SaveMessage 方法）
 	if s.conversationCache != nil {
-		seq, err := s.conversationCache.GetNextSeq(ctx, conversationID)
+		seq, err := s.conversationCache.GetNextSeqWithGroup(ctx, conversationID, conv.Type == model.ConversationTypeGroup)
 		if err != nil {
 			return nil, fmt.Errorf("failed to get next seq for conversation %s: %w", conversationID, err)
 		}