feat(distribution): 支持分站发布群固定二维码

主站分销配置可为每个站点绑定可复用企微码，号主在该站上传账号后发到对应发布群，不再消耗主站码池。

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

backend/internal/modules/chat/listing_group.go

@@ -38,6 +38,11 @@ func EnsureListingConversation(tx *gorm.DB, listing model.RentalListing, preferr
 		return nil, err
 	}
 
+	siteID, err := resolveListingDistributionSiteID(tx, listing)
+	if err != nil {
+		return nil, err
+	}
+
 	// 2. 建会话
 	now := time.Now()
 	conversation := model.ChatConversation{
@@ -46,6 +51,9 @@ func EnsureListingConversation(tx *gorm.DB, listing model.RentalListing, preferr
 		Title:     listingConversationTitle(listing),
 		Status:    "active",
 	}
+	if siteID > 0 {
+		conversation.DistributionSiteID = &siteID
+	}
 	if err := tx.Create(&conversation).Error; err != nil {
 		return nil, err
 	}
@@ -88,17 +96,12 @@ func EnsureListingConversation(tx *gorm.DB, listing model.RentalListing, preferr
 		}
 	}
 
-	// 4. 取二维码(带行锁)
-	var qrcode *model.ChatQrCode
-	var qrcodeImageURL string
-	qrcodeObj, err := fetchUnusedQrCodeForListing(tx, listing.ID)
+	// 4. 取二维码：分站优先用站点固定码（可复用）；否则消耗对应码池。
+	qrcode, qrcodeImageURL, err := resolveListingQrCode(tx, siteID)
 	if err != nil {
 		return nil, err
 	}
-	if qrcodeObj != nil {
-		qrcode = qrcodeObj
-		qrcodeImageURL = qrcode.ImageURL
-		// 标记二维码为已使用
+	if qrcode != nil {
 		if err := markQrCodeAsUsed(tx, qrcode.ID, conversation.ID); err != nil {
 			return nil, err
 		}
@@ -309,22 +312,60 @@ func fetchUnusedQrCode(tx *gorm.DB) (*model.ChatQrCode, error) {
 	return fetchUnusedQrCodeForSite(tx, nil)
 }
 
-func fetchUnusedQrCodeForListing(tx *gorm.DB, listingID uint64) (*model.ChatQrCode, error) {
+func resolveListingDistributionSiteID(tx *gorm.DB, listing model.RentalListing) (uint64, error) {
+	if listing.DistributionSiteID != nil && *listing.DistributionSiteID > 0 {
+		return *listing.DistributionSiteID, nil
+	}
+	if listing.ID == 0 {
+		return 0, nil
+	}
 	var relation model.DistributionSiteAccount
 	// 仅匹配启用中的分销站点，真实 DB 错误直接返回，避免误回退到全局二维码池。
 	err := tx.Table("distribution_site_accounts AS relation").
 		Select("relation.*").
 		Joins("JOIN distribution_sites AS site ON site.id = relation.site_id AND site.status = 'active'").
-		Where("relation.listing_id = ? AND relation.display_status = 'enabled'", listingID).
+		Where("relation.listing_id = ? AND relation.display_status = 'enabled'", listing.ID).
 		Order("relation.id ASC").
+		Limit(1).
 		Scan(&relation).Error
 	if err != nil {
-		return nil, err
+		return 0, err
 	}
 	if relation.ID != 0 {
-		return fetchUnusedQrCodeForSite(tx, &relation.SiteID)
+		return relation.SiteID, nil
 	}
-	return fetchUnusedQrCodeForSite(tx, nil)
+	return 0, nil
+}
+
+func resolveListingQrCode(tx *gorm.DB, siteID uint64) (*model.ChatQrCode, string, error) {
+	if siteID > 0 {
+		var site model.DistributionSite
+		err := tx.Select("id", "listing_qrcode_url").First(&site, siteID).Error
+		if err != nil && !errors.Is(err, gorm.ErrRecordNotFound) {
+			return nil, "", err
+		}
+		if err == nil {
+			if url := strings.TrimSpace(site.ListingQrcodeURL); url != "" {
+				return nil, url, nil
+			}
+		}
+		qrcode, err := fetchUnusedQrCodeForSite(tx, &siteID)
+		if err != nil {
+			return nil, "", err
+		}
+		if qrcode != nil {
+			return qrcode, qrcode.ImageURL, nil
+		}
+		return nil, "", nil
+	}
+	qrcode, err := fetchUnusedQrCodeForSite(tx, nil)
+	if err != nil {
+		return nil, "", err
+	}
+	if qrcode != nil {
+		return qrcode, qrcode.ImageURL, nil
+	}
+	return nil, "", nil
 }
 
 func fetchUnusedQrCodeForSite(tx *gorm.DB, siteID *uint64) (*model.ChatQrCode, error) {