refactor(messaging): shift sequence generation responsibility to database transactions

Move the responsibility of sequence (`seq`) allocation from the application/cache layer to the database layer to ensure absolute consistency. Previously, sequences were pre-allocated via Redis/Lua, which introduced complexity in managing synchronization between the cache and the database.

Key changes:
- Implement `CreateMessageWithSeq` in `message_repo.go` using `SELECT FOR UPDATE` to lock the conversation row and atomically increment the `last_seq` within a single transaction.
- Simplify `SeqBufferManager` by removing the complex local buffering and Lua-based pre-allocation logic, reverting to a simpler model.
- Update `chat_service.go`, `group_service.go`, and `message_service.go` to remove manual sequence retrieval, relying instead on the repository's transactional allocation.
- Introduce `SyncConvSeq` in `conversation_cache.go` to perform "write-through" updates to Redis, ensuring the cache remains synchronized with the database's source of truth.
- Improve cold-start handling in `conversation_cache.go` with a new `syncSeqLua` script to prevent stale sequence reads.

internal/service/chat_service.go

@@ -479,20 +479,18 @@ func (s *chatServiceImpl) SendMessage(ctx context.Context, senderID string, conv
 		Status:         model.MessageStatusNormal,
 	}
 
-	// 从 Redis 获取下一个 seq
-	if s.conversationCache != nil {
-		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)
-		}
-		message.Seq = seq
-	}
+		// seq 由 CreateMessageWithSeq 在事务内原子分配
 
 	// 使用事务创建消息并更新seq
 	if err := s.repo.CreateMessageWithSeq(message); err != nil {
 		return nil, fmt.Errorf("failed to save message: %w", err)
 	}
 
+	// 写透 seq 到 Redis，保持缓存与 DB 一致
+	if s.conversationCache != nil {
+		s.conversationCache.SyncConvSeq(context.Background(), conversationID, message.Seq)
+	}
+
 	// 发送者自动已读：将发送者的 lastReadSeq 更新到消息 seq
 	_ = s.repo.UpdateLastReadSeq(conversationID, senderID, message.Seq)
 	if s.conversationCache != nil {
@@ -1042,7 +1040,7 @@ 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
-	conv, err := s.getConversation(ctx, conversationID)
+	_, 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")
@@ -1073,19 +1071,17 @@ func (s *chatServiceImpl) SaveMessage(ctx context.Context, senderID string, conv
 		Status:         model.MessageStatusNormal,
 	}
 
-	// 从 Redis 获取下一个 seq（SaveMessage 方法）
-	if s.conversationCache != nil {
-		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)
-		}
-		message.Seq = seq
-	}
+		// seq 由 CreateMessageWithSeq 在事务内原子分配
 
 	if err := s.repo.CreateMessageWithSeq(message); err != nil {
 		return nil, fmt.Errorf("failed to save message: %w", err)
 	}
 
+	// 写透 seq 到 Redis，保持缓存与 DB 一致
+	if s.conversationCache != nil {
+		s.conversationCache.SyncConvSeq(context.Background(), conversationID, message.Seq)
+	}
+
 	// 新消息会改变分页结果，先失效分页缓存，避免读到旧列表
 	if s.conversationCache != nil {
 		s.conversationCache.InvalidateMessagePages(conversationID)