feat(chat): implement sequence-based unread count tracking

Introduce a new mechanism for tracking read positions using message
sequences (hasReadSeq), similar to OpenIM, to allow for O(1) unread
count calculations.

- Implement `UserReadSeq` caching in Redis with Lua scripts to prevent
  sequence regression (ensuring updates only occur if the new sequence
  is greater than the current one).
- Update `ConversationCache` to support sequence-based unread count
  computation (`maxSeq - hasReadSeq`).
- Refactor `MessageRepository` to use conditional updates for
  `last_read_seq` in the database.
- Update `ChatService` to automatically update the sender's read
  sequence when sending a message.
- Optimize `MarkAsRead` logic to update both database and Redis
  sequence caches and refine notification targets for private vs
  group chats.
- Update `jpush` client to use pointer types for boolean fields to
  properly handle optional values in JSON.

internal/repository/message_repo.go

@@ -278,27 +278,41 @@ func (r *messageRepository) GetParticipant(conversationID string, userID string)
 	return &participant, nil
 }
 
-// UpdateLastReadSeq 更新已读位置
+// UpdateLastReadSeq 更新已读位置（防回退：仅当新值大于当前值时才更新）
 // userID 参数为 string 类型（UUID格式），与JWT中user_id保持一致
 func (r *messageRepository) UpdateLastReadSeq(conversationID string, userID string, lastReadSeq int64) error {
+	// 使用条件更新防止已读位置回退
 	result := r.db.Model(&model.ConversationParticipant{}).
-		Where("conversation_id = ? AND user_id = ?", conversationID, userID).
+		Where("conversation_id = ? AND user_id = ? AND last_read_seq < ?", conversationID, userID, lastReadSeq).
 		Update("last_read_seq", lastReadSeq)
 
 	if result.Error != nil {
 		return result.Error
 	}
 
-	// 如果没有更新任何记录，说明参与者记录不存在，需要插入
+	// 如果没有更新任何记录，可能是参与者记录不存在，尝试插入
 	if result.RowsAffected == 0 {
-		// 尝试插入新记录（跨数据库 upsert）
-		err := r.db.Clauses(clause.OnConflict{
+		// 检查是否已有记录但 seq 不需要更新
+		var existingSeq int64
+		err := r.db.Model(&model.ConversationParticipant{}).
+			Where("conversation_id = ? AND user_id = ?", conversationID, userID).
+			Select("last_read_seq").Scan(&existingSeq).Error
+		if err == nil && existingSeq >= lastReadSeq {
+			// 已有记录且 seq 不需要回退，直接返回成功
+			return nil
+		}
+		if err != nil && !errors.Is(err, gorm.ErrRecordNotFound) {
+			return err
+		}
+
+		// 参与者记录不存在，插入新记录
+		err = r.db.Clauses(clause.OnConflict{
 			Columns: []clause.Column{
 				{Name: "conversation_id"},
 				{Name: "user_id"},
 			},
 			DoUpdates: clause.Assignments(map[string]any{
-				"last_read_seq": lastReadSeq,
+				"last_read_seq": gorm.Expr("CASE WHEN last_read_seq < ? THEN ? ELSE last_read_seq END", lastReadSeq, lastReadSeq),
 				"updated_at":    gorm.Expr("CURRENT_TIMESTAMP"),
 			}),
 		}).Create(&model.ConversationParticipant{