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/repository/group_repo.go

@@ -27,6 +27,7 @@ type GroupRepository interface {
 	UpdateMember(member *model.GroupMember) error
 	RemoveMember(groupID string, userID string) error
 	GetMemberCount(groupID string) (int64, error)
+	GetMemberCountBatch(ctx context.Context, groupIDs []string) (map[string]int64, error)
 	IsMember(groupID string, userID string) (bool, error)
 	GetUserGroups(userID string, page, pageSize int) ([]model.Group, int64, error)
 
@@ -225,6 +226,34 @@ func (r *groupRepository) GetMemberCount(groupID string) (int64, error) {
 	return count, err
 }
 
+// GetMemberCountBatch 批量获取多个群的成员数量
+func (r *groupRepository) GetMemberCountBatch(ctx context.Context, groupIDs []string) (map[string]int64, error) {
+	result := make(map[string]int64, len(groupIDs))
+	if len(groupIDs) == 0 {
+		return result, nil
+	}
+
+	type row struct {
+		GroupID     string
+		Count       int64
+	}
+	var rows []row
+	err := r.db.WithContext(ctx).
+		Model(&model.GroupMember{}).
+		Select("group_id, COUNT(*) as count").
+		Where("group_id IN ?", groupIDs).
+		Group("group_id").
+		Scan(&rows).Error
+	if err != nil {
+		return nil, err
+	}
+
+	for _, r := range rows {
+		result[r.GroupID] = r.Count
+	}
+	return result, nil
+}
+
 // IsMember 检查是否是群成员
 func (r *groupRepository) IsMember(groupID string, userID string) (bool, error) {
 	var count int64