feat(im): Task 4 - WS 事件处理器 + 离线消息推送

EventHandler:
- im.message.send: 发送消息（解析→业务→ACK+推送）
- im.message.recall: 撤回消息
- im.conversation.read: 标记已读
- im.typing: 正在输入通知（转发对方）
- 所有事件通过 Hub.RegisterEvent 注册到路由表

OfflinePusher:
- PushOfflineMessages: 推送未读会话摘要 + 全局未读总数
- 通过 OfflineMessagePusher 接口注入 WS Handler
- WebSocket 连接建立后自动触发离线推送

Made-with: Cursor

backend/go-service/app/im/handler/offline_pusher.go

@@ -0,0 +1,84 @@
+package handler
+
+import (
+	"context"
+
+	"github.com/echochat/backend/app/im/dao"
+	"github.com/echochat/backend/app/im/service"
+	"github.com/echochat/backend/pkg/logs"
+	"github.com/echochat/backend/pkg/ws"
+	"go.uber.org/zap"
+)
+
+// OfflinePusher 离线消息推送器
+// 在用户 WebSocket 重新连接后，主动推送未读会话摘要
+type OfflinePusher struct {
+	imService *service.IMService
+	convDAO   *dao.ConversationDAO
+	pubsub    *ws.PubSub
+}
+
+// NewOfflinePusher 创建 OfflinePusher 实例
+func NewOfflinePusher(imService *service.IMService, convDAO *dao.ConversationDAO, pubsub *ws.PubSub) *OfflinePusher {
+	return &OfflinePusher{
+		imService: imService,
+		convDAO:   convDAO,
+		pubsub:    pubsub,
+	}
+}
+
+// PushOfflineMessages 向用户推送离线消息摘要
+// 包含所有有未读消息的会话信息和全局未读总数
+func (p *OfflinePusher) PushOfflineMessages(ctx context.Context, userID int64) {
+	funcName := "handler.offline_pusher.PushOfflineMessages"
+	logs.Info(ctx, funcName, "推送离线消息", zap.Int64("user_id", userID))
+
+	unreadConvs, err := p.convDAO.GetUnreadConversations(ctx, userID)
+	if err != nil {
+		logs.Error(ctx, funcName, "查询未读会话失败", zap.Error(err))
+		return
+	}
+
+	totalUnread, err := p.imService.GetTotalUnread(ctx, userID)
+	if err != nil {
+		logs.Error(ctx, funcName, "查询全局未读数失败", zap.Error(err))
+		totalUnread = 0
+	}
+
+	if len(unreadConvs) == 0 && totalUnread == 0 {
+		return
+	}
+
+	convList := make([]map[string]interface{}, 0, len(unreadConvs))
+	for _, c := range unreadConvs {
+		item := map[string]interface{}{
+			"conversation_id": c.ID,
+			"unread_count":    c.UnreadCount,
+			"last_msg_content": c.LastMsgContent,
+		}
+		if c.LastMsgTime != nil {
+			item["last_msg_time"] = c.LastMsgTime.Format("2006-01-02 15:04:05")
+		}
+		convList = append(convList, item)
+	}
+
+	push := ws.NewPushMessage("im.offline.sync", map[string]interface{}{
+		"total_unread":  totalUnread,
+		"conversations": convList,
+	})
+	data, err := ws.MarshalPush(push)
+	if err != nil {
+		logs.Error(ctx, funcName, "序列化离线消息失败", zap.Error(err))
+		return
+	}
+
+	if err := p.pubsub.Publish(ctx, userID, data); err != nil {
+		logs.Error(ctx, funcName, "推送离线消息失败",
+			zap.Int64("user_id", userID), zap.Error(err))
+	}
+
+	logs.Info(ctx, funcName, "离线消息推送完成",
+		zap.Int64("user_id", userID),
+		zap.Int("unread_conv_count", len(unreadConvs)),
+		zap.Int64("total_unread", totalUnread))
+}