feat: 添加媒体资产变体和来源ID字段，支持预览产品与主资产的关联

backend/app/services/content.go

@@ -34,6 +34,16 @@ type ContentDetailResult struct {
 	HasAccess bool
 }
 
+func requiredMediaAssetVariantForRole(role consts.ContentAssetRole) string {
+	switch role {
+	case consts.ContentAssetRolePreview:
+		return mediaAssetVariantPreview
+	default:
+		// main/cover 一律要求 main 产物，避免误把 preview 绑定成正片/封面。
+		return mediaAssetVariantMain
+	}
+}
+
 func (s *content) Create(ctx context.Context, tenantID, userID int64, form *dto.ContentCreateForm) (*models.Content, error) {
 	log.WithFields(log.Fields{
 		"tenant_id": tenantID,
@@ -215,6 +225,60 @@ func (s *content) AttachAsset(ctx context.Context, tenantID, userID, contentID,
 		return nil, errorx.ErrPreconditionFailed.WithMsg("media asset not ready")
 	}
 
+	// C2 规则：preview 必须绑定独立产物（media_assets.variant=preview），main/cover 必须为 main。
+	var assetRow struct {
+		Variant       string `gorm:"column:variant"`
+		SourceAssetID *int64 `gorm:"column:source_asset_id"`
+	}
+	if err := _db.WithContext(ctx).
+		Table(models.TableNameMediaAsset).
+		Select("variant, source_asset_id").
+		Where("tenant_id = ? AND id = ? AND deleted_at IS NULL", tenantID, assetID).
+		Take(&assetRow).Error; err != nil {
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			return nil, errorx.ErrRecordNotFound.WithMsg("media asset not found")
+		}
+		return nil, err
+	}
+	variant := assetRow.Variant
+	if variant == "" {
+		variant = mediaAssetVariantMain
+	}
+	requiredVariant := requiredMediaAssetVariantForRole(role)
+	if variant != requiredVariant {
+		return nil, errorx.ErrPreconditionFailed.WithMsg("media asset variant mismatch")
+	}
+	// 关联规则：preview 产物必须声明来源 main；main/cover 不允许带来源。
+	if role == consts.ContentAssetRolePreview {
+		if assetRow.SourceAssetID == nil || *assetRow.SourceAssetID <= 0 {
+			return nil, errorx.ErrPreconditionFailed.WithMsg("preview asset must have source_asset_id")
+		}
+		var srcRow struct {
+			Variant string `gorm:"column:variant"`
+		}
+		if err := _db.WithContext(ctx).
+			Table(models.TableNameMediaAsset).
+			Select("variant").
+			Where("tenant_id = ? AND id = ? AND deleted_at IS NULL", tenantID, *assetRow.SourceAssetID).
+			Take(&srcRow).Error; err != nil {
+			if errors.Is(err, gorm.ErrRecordNotFound) {
+				return nil, errorx.ErrRecordNotFound.WithMsg("preview source asset not found")
+			}
+			return nil, err
+		}
+		srcVariant := srcRow.Variant
+		if srcVariant == "" {
+			srcVariant = mediaAssetVariantMain
+		}
+		if srcVariant != mediaAssetVariantMain {
+			return nil, errorx.ErrPreconditionFailed.WithMsg("preview source asset must be main variant")
+		}
+	} else {
+		if assetRow.SourceAssetID != nil && *assetRow.SourceAssetID > 0 {
+			return nil, errorx.ErrPreconditionFailed.WithMsg("main/cover asset must not have source_asset_id")
+		}
+	}
+
 	m := &models.ContentAsset{
 		TenantID:  tenantID,
 		UserID:    userID,