fix(chat): 撤回消息时回退未读角标，避免当成新消息

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/handler/chat.go

@@ -401,12 +401,12 @@ func (h *Handlers) RecallChatMessage(c *gin.Context) {
 		return
 	}
 	oversee := h.chatOversee(claims.ID)
-	msg, err := h.Chat.RecallMessage(claims.ID, roomID, uint(mid), oversee)
+	msg, newly, err := h.Chat.RecallMessage(claims.ID, roomID, uint(mid), oversee)
 	if err != nil {
 		c.JSON(chatErrToStatus(err), gin.H{"error": err.Error()})
 		return
 	}
-	h.broadcastChatRecalled(roomID, msg)
+	h.broadcastChatRecalled(roomID, msg, newly)
 	// 监管者保留正文供界面展示；撤回者本人保留正文供「重新编辑」；其余不回传
 	if !oversee && msg.SenderID != claims.ID {
 		msg.Content = ""
@@ -504,19 +504,19 @@ func (h *Handlers) AdminRecallChatMessage(c *gin.Context) {
 	}
 	oversee := h.chatOversee(claims.ID)
 	// 非监管 staff：仅允许撤回自己发送的（与前台一致）
-	msg, err := h.Chat.RecallMessage(claims.ID, uint(roomID), uint(mid), oversee)
+	msg, newly, err := h.Chat.RecallMessage(claims.ID, uint(roomID), uint(mid), oversee)
 	if err != nil {
 		c.JSON(chatErrToStatus(err), gin.H{"error": err.Error()})
 		return
 	}
-	h.broadcastChatRecalled(uint(roomID), msg)
+	h.broadcastChatRecalled(uint(roomID), msg, newly)
 	c.JSON(http.StatusOK, gin.H{
 		"ok": true, "id": msg.ID, "room_id": roomID, "recalled_at": msg.RecalledAt,
 	})
 }
 
 // broadcastChatRecalled 会话内撤回与管理端撤回共用：先落库再推 chat:{roomID}
-func (h *Handlers) broadcastChatRecalled(roomID uint, msg *model.ChatMessage) {
+func (h *Handlers) broadcastChatRecalled(roomID uint, msg *model.ChatMessage, newly bool) {
 	if h.Hub == nil || msg == nil {
 		return
 	}
@@ -547,4 +547,35 @@ func (h *Handlers) broadcastChatRecalled(roomID uint, msg *model.ChatMessage) {
 			h.Hub.BroadcastUser(uid, env)
 		}
 	}
+	// 发送时已给未读者 +1；撤回后必须回退，否则角标会停在一条已撤回的「新消息」上
+	if newly {
+		h.broadcastChatUnreadDecrement(roomID, msg)
+	}
+}
+
+// broadcastChatUnreadDecrement 与发信对称：给除发送者外的全体成员推 delta:-1。
+// 不能按 last_read 过滤——发信时 +1 也是全员推的；另一标签页已读后首页角标仍需回退。
+func (h *Handlers) broadcastChatUnreadDecrement(roomID uint, msg *model.ChatMessage) {
+	if h.Hub == nil || h.Chat == nil || msg == nil {
+		return
+	}
+	ids, err := h.Chat.ListMemberIDs(roomID)
+	if err != nil {
+		return
+	}
+	env := realtime.Envelope{
+		Type: realtime.EventChatUnread,
+		Data: gin.H{
+			"room_id":    roomID,
+			"message_id": msg.ID,
+			"sender_id":  msg.SenderID,
+			"delta":      -1,
+		},
+	}
+	for _, uid := range ids {
+		if uid == msg.SenderID {
+			continue
+		}
+		h.Hub.BroadcastUser(uid, env)
+	}
 }