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/handler/message_handler.go

@@ -565,6 +565,26 @@ func (h *MessageHandler) GetUnreadCount(c *gin.Context) {
 	})
 }
 
+// HandleGetSyncData 获取同步元数据（轻量级 seq + 时间）
+// GET /api/v1/conversations/sync-data
+func (h *MessageHandler) HandleGetSyncData(c *gin.Context) {
+	userID := c.GetString("user_id")
+	if userID == "" {
+		response.Unauthorized(c, "")
+		return
+	}
+
+	items, err := h.chatService.GetSyncData(c.Request.Context(), userID)
+	if err != nil {
+		response.InternalServerError(c, "failed to get sync data")
+		return
+	}
+
+	response.Success(c, &dto.SyncDataResponse{
+		Conversations: items,
+	})
+}
+
 // GetConversationUnreadCount 获取单个会话的未读数
 // GET /api/conversations/:id/unread/count
 func (h *MessageHandler) GetConversationUnreadCount(c *gin.Context) {
@@ -900,6 +920,33 @@ func (h *MessageHandler) HandleMarkRead(c *gin.Context) {
 	response.SuccessWithMessage(c, "marked as read", nil)
 }
 
+// HandleMarkReadAll 批量标记所有会话已读
+// POST /api/v1/conversations/read-all
+func (h *MessageHandler) HandleMarkReadAll(c *gin.Context) {
+	userID := c.GetString("user_id")
+	if userID == "" {
+		response.Unauthorized(c, "")
+		return
+	}
+
+	var req dto.BatchMarkReadRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		response.BadRequest(c, err.Error())
+		return
+	}
+
+	successCount, err := h.chatService.MarkAsReadBatch(c.Request.Context(), userID, req.Conversations)
+	if err != nil {
+		response.InternalServerError(c, "failed to mark all as read")
+		return
+	}
+
+	response.Success(c, &dto.BatchMarkReadResponse{
+		SuccessCount: successCount,
+		TotalCount:   len(req.Conversations),
+	})
+}
+
 // HandleSetConversationPinned 设置会话置顶
 // PUT /api/v1/conversations/:id/pinned
 func (h *MessageHandler) HandleSetConversationPinned(c *gin.Context) {