feat(chat): implement batch mark-as-read and message sync data

Refactor unread count logic to use arithmetic calculation (maxSeq - readSeq)
instead of manual incrementing/decrementing. This simplifies the cache
management and improves consistency.

New features:
- Batch mark-as-read functionality for multiple conversations.
- Lightweight sync data endpoint to retrieve conversation metadata
  (maxSeq and last message timestamp) for client synchronization.
- Optimized batch retrieval of conversation max sequences using Redis MGet.

Technical changes:
- Deprecated `IncrementUnread` and `ClearUnread` in `ConversationCache`.
- Added `GetConvMaxSeqBatch` to reduce network round-trips.
- Added `HandleMarkReadAll` and `HandleGetSyncData` handlers.
- Updated `ChatService` to support batch operations and sync data retrieval.

internal/router/router.go

@@ -426,9 +426,11 @@ func (r *Router) setupRoutes() {
 			conversations.GET("/:id/messages/cursor", r.messageHandler.GetMessagesByCursor)                                 // 消息列表游标分页
 			conversations.POST("/:id/messages", middleware.RequireVerified(r.userRepo), r.messageHandler.HandleSendMessage) // 发送消息
 			conversations.POST("/:id/read", r.messageHandler.HandleMarkRead)                                                // 标记已读
+				conversations.POST("/read-all", r.messageHandler.HandleMarkReadAll)                                                 // 批量标记已读
 			conversations.PUT("/:id/pinned", r.messageHandler.HandleSetConversationPinned)                                  // 置顶设置
 			conversations.PUT("/:id/notification_muted", r.messageHandler.HandleSetConversationNotificationMuted)          // 免打扰设置
 			conversations.GET("/unread/count", r.messageHandler.GetUnreadCount)                                             // 未读数
+				conversations.GET("/sync-data", r.messageHandler.HandleGetSyncData)                                                    // 同步元数据
 			conversations.POST("/:id/typing", r.messageHandler.HandleTyping)                                                // 输入状态
 			conversations.DELETE("/:id/self", r.messageHandler.HandleDeleteConversationForSelf)                             // 删除会话(仅自己)