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)

internal/service/group_service.go

@@ -471,18 +471,7 @@ func (s *groupService) broadcastMemberJoinNotice(groupID string, targetUserID st
 				Status:   model.MessageStatusNormal,
 				Category: model.CategoryNotification,
 			}
-			// 从 Redis 获取下一个 seq
-			if s.conversationCache != nil {
-				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),
-						zap.Error(seqErr),
-					)
-					return
-				}
-				msg.Seq = seq
-			}
+			// seq 由 CreateMessageWithSeq 在事务内原子分配
 			if err := s.messageRepo.CreateMessageWithSeq(msg); err != nil {
 				zap.L().Warn("保存入群提示消息失败",
 					zap.String("component", "broadcastMemberJoinNotice"),
@@ -492,6 +481,9 @@ func (s *groupService) broadcastMemberJoinNotice(groupID string, targetUserID st
 				)
 			} else {
 				savedMessage = msg
+				if s.conversationCache != nil {
+					s.conversationCache.SyncConvSeq(context.Background(), conv.ID, msg.Seq)
+				}
 				s.invalidateConversationCachesAfterSystemMessage(conv.ID)
 			}
 		} else {
@@ -1396,18 +1388,7 @@ func (s *groupService) MuteMember(userID string, groupID string, targetUserID st
 				Category: model.CategoryNotification,
 			}
 
-			// 从 Redis 获取下一个 seq
-			if s.conversationCache != nil {
-				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),
-						zap.Error(seqErr),
-					)
-					return nil
-				}
-				msg.Seq = seq
-			}
+			// seq 由 CreateMessageWithSeq 在事务内原子分配
 
 			if err := s.messageRepo.CreateMessageWithSeq(msg); err != nil {
 				zap.L().Warn("保存禁言消息失败",
@@ -1421,6 +1402,9 @@ func (s *groupService) MuteMember(userID string, groupID string, targetUserID st
 					zap.String("msgID", msg.ID),
 					zap.Int64("seq", msg.Seq),
 				)
+				if s.conversationCache != nil {
+					s.conversationCache.SyncConvSeq(context.Background(), conv.ID, msg.Seq)
+				}
 				s.invalidateConversationCachesAfterSystemMessage(conv.ID)
 			}
 		} else {

internal/service/message_service.go

@@ -2,7 +2,6 @@ package service
 
 import (
 	"context"
-	"fmt"
 	"time"
 
 	"with_you/internal/cache"
@@ -88,21 +87,17 @@ func (s *MessageService) SendMessage(ctx context.Context, senderID, receiverID s
 		Status:         model.MessageStatusNormal,
 	}
 
-	// 从 Redis 获取下一个 seq
-	if s.conversationCache != nil {
-		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)
-		}
-		msg.Seq = seq
-	}
-
-	// 使用事务创建消息并更新seq
+	// 事务内原子分配 seq + 创建消息（DB 是 seq 唯一来源）
 	err = s.messageRepo.CreateMessageWithSeq(msg)
 	if err != nil {
 		return nil, err
 	}
 
+	// 写透 seq 到 Redis，保持缓存与 DB 一致
+	if s.conversationCache != nil {
+		s.conversationCache.SyncConvSeq(context.Background(), conv.ID, msg.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(conv.ID)