feat: 封存解散群聊后用户端隐藏 archived 会话

- 用户会话列表(ListConversations)过滤 status=active,死群不再堆在列表
- 未读统计(CountUnreadMessages)排除 archived 会话,消除消不掉的红点
- 数据完整保留,客服后台与历史记录查看不受影响,保留纠纷证据链

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/internal/modules/chat/conversation.go

@@ -15,7 +15,8 @@ func (r *Repository) ListConversations(ctx context.Context, principal Principal,
 	var total int64
 	countDB := db.Table("chat_conversations AS c").
 		Joins("JOIN chat_participants AS cp ON cp.conversation_id = c.id").
-		Where("cp.participant_type = ? AND cp.participant_id = ?", principal.Type, principal.ID)
+		Where("cp.participant_type = ? AND cp.participant_id = ?", principal.Type, principal.ID).
+		Where("c.status = ?", "active")
 	if err := countDB.Count(&total).Error; err != nil {
 		return nil, err
 	}
@@ -23,6 +24,7 @@ func (r *Repository) ListConversations(ctx context.Context, principal Principal,
 	var rows []conversationRow
 	offset := (page - 1) * pageSize
 	err := r.conversationQuery(ctx, principal).
+		Where("c.status = ?", "active").
 		Order("COALESCE(c.last_message_at, c.created_at) DESC, c.id DESC").
 		Offset(offset).
 		Limit(pageSize).

backend/internal/modules/chat/participant.go

@@ -54,7 +54,9 @@ func (r *Repository) CountUnreadMessages(ctx context.Context, principal Principa
 	var total int64
 	err := r.db.WithContext(ctx).Table("chat_messages AS cm").
 		Joins("JOIN chat_participants AS cp ON cp.conversation_id = cm.conversation_id").
+		Joins("JOIN chat_conversations AS c ON c.id = cm.conversation_id").
 		Where("cp.participant_type = ? AND cp.participant_id = ?", principal.Type, principal.ID).
+		Where("c.status = ?", "active").
 		Where("NOT (cm.sender_type = ? AND cm.sender_id = ?)", principal.Type, principal.ID).
 		Where("(cp.last_read_at IS NULL OR cm.created_at > cp.last_read_at)").
 		Count(&total).Error