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/dto/dto.go

@@ -437,6 +437,23 @@ type MarkReadRequest struct {
 	LastReadSeq int64 `json:"last_read_seq" binding:"required"` // 已读到的seq位置
 }
 
+// BatchMarkReadItem 批量标记已读单项
+type BatchMarkReadItem struct {
+	ConversationID string `json:"conversation_id" binding:"required"`
+	LastReadSeq    int64  `json:"last_read_seq" binding:"required"`
+}
+
+// BatchMarkReadRequest 批量标记已读请求
+type BatchMarkReadRequest struct {
+	Conversations []BatchMarkReadItem `json:"conversations" binding:"required"`
+}
+
+// BatchMarkReadResponse 批量标记已读响应
+type BatchMarkReadResponse struct {
+	SuccessCount int `json:"success_count"`
+	TotalCount   int `json:"total_count"`
+}
+
 // SetConversationPinnedRequest 设置会话置顶请求
 type SetConversationPinnedRequest struct {
 	ConversationID string `json:"conversation_id" binding:"required"`
@@ -481,6 +498,18 @@ type ConversationUnreadCountResponse struct {
 	UnreadCount    int64  `json:"unread_count"`
 }
 
+// SyncDataItem 同步数据单项（轻量级，仅包含 seq 和时间）
+type SyncDataItem struct {
+	ConversationID string `json:"id"`
+	MaxSeq         int64  `json:"max_seq"`
+	LastMessageAt  string `json:"last_message_at"`
+}
+
+// SyncDataResponse 同步数据响应
+type SyncDataResponse struct {
+	Conversations []SyncDataItem `json:"conversations"`
+}
+
 // MessageListResponse 消息列表响应
 type MessageListResponse struct {
 	Messages []*MessageResponse `json:"messages"`