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/handler/message_handler.go

@@ -758,7 +758,7 @@ func (h *MessageHandler) HandleGetConversation(c *gin.Context) {
 	// 获取参与者信息
 	participants, _ := h.getConversationParticipants(c.Request.Context(), conversationID, userID)
 
-	// 获取当前用户的已读位置
+	// 获取当前用户的已读位置（优先从 Redis 缓存读取）
 	myLastReadSeq := int64(0)
 	isPinned := false
 	notificationMuted := false
@@ -772,7 +772,7 @@ func (h *MessageHandler) HandleGetConversation(c *gin.Context) {
 		}
 	}
 
-	// 获取对方用户的已读位置
+	// 获取对方用户的已读位置（优先从 Redis 缓存读取）
 	otherLastReadSeq := int64(0)
 	for _, p := range allParticipants {
 		if p.UserID != userID {