feat(core): optimize performance and reliability through batching and redis-backed unread counts

This commit introduces several significant architectural improvements to enhance system performance, scalability, and reliability:

- **Performance Optimization (N+1 Problem Resolution)**: Implemented batching mechanisms across `MessageHandler`, `ChatService`, `GroupService`, `MessageService`, and `UserService`. This replaces multiple individual database/cache queries with single batch operations for fetching unread counts, last messages, participants, and user information.
- **Enhanced Unread Count Management**: Migrated unread count tracking to a Redis Hash-based approach (`unread#️⃣{userID}`). This allows for atomic increments/decrements and efficient retrieval of both individual conversation unread counts and total unread counts for a user.
- **Improved Message Sequencing**: Integrated Redis-based sequence (`seq`) pre-allocation for messages to ensure strict ordering and reduce database contention during high-concurrency message creation.
- **Push Service Reliability**: Refactored the push notification system to include persistent `PushRecord` tracking. Added a recovery mechanism (`recoverPendingPushes`) to reload pending notifications from the database upon service startup, ensuring better delivery guarantees.
- **WebSocket Reliability**: Updated the WebSocket hub to move away from in-memory history replay in favor of a client-driven synchronization model (`sync_required` event and `ack` handling), reducing memory overhead and improving connection stability.
- **Cache Layer Enhancements**: Added `HIncrBy` and `IncrBySeq` to the `Cache` interface and its implementations (`RedisCache`, `LayeredCache`) to support the new unread and sequence management logic.

internal/service/message_service.go

@@ -86,6 +86,19 @@ 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.GetNextSeq(context.Background(), conv.ID)
+		if seqErr != nil {
+			zap.L().Warn("redis get next seq failed, falling back to DB",
+				zap.String("convID", conv.ID),
+				zap.Error(seqErr),
+			)
+		} else {
+			msg.Seq = seq
+		}
+	}
+
 	// 使用事务创建消息并更新seq
 	err = s.messageRepo.CreateMessageWithSeq(msg)
 	if err != nil {
@@ -284,6 +297,16 @@ func (s *MessageService) GetConversationParticipants(conversationID string) ([]*
 	return s.messageRepo.GetConversationParticipants(conversationID)
 }
 
+// GetParticipantsBatch 批量获取多个会话的参与者列表
+func (s *MessageService) GetParticipantsBatch(ctx context.Context, convIDs []string) (map[string][]*model.ConversationParticipant, error) {
+	return s.messageRepo.GetParticipantsBatch(ctx, convIDs)
+}
+
+// GetMyParticipantsBatch 批量获取用户在多个会话中的参与者信息
+func (s *MessageService) GetMyParticipantsBatch(ctx context.Context, convIDs []string, userID string) (map[string]*model.ConversationParticipant, error) {
+	return s.messageRepo.GetMyParticipantsBatch(ctx, convIDs, userID)
+}
+
 // ParseConversationID 辅助函数：直接返回字符串ID（已经是string类型）
 func ParseConversationID(idStr string) (string, error) {
 	return idStr, nil