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/http/tenant/dto/media_asset_admin.go

@@ -7,6 +7,14 @@ type AdminMediaAssetUploadInitForm struct {
 	// Type is the media asset type (video/audio/image).
 	// Used to decide processing pipeline and validation rules; required.
 	Type string `json:"type,omitempty"`
+
+	// Variant indicates whether this asset is a main or preview product.
+	// Allowed: main/preview; default is main.
+	Variant string `json:"variant,omitempty"`
+
+	// SourceAssetID links a preview product to its main asset; only meaningful when variant=preview.
+	SourceAssetID *int64 `json:"source_asset_id,omitempty"`
+
 	// ContentType is the MIME type reported by the client (e.g. video/mp4); optional.
 	// Server should not fully trust it, but can use it as a hint for validation/logging.
 	ContentType string `json:"content_type,omitempty"`

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,

backend/app/services/media_asset.go

@@ -31,6 +31,11 @@ import (
 //	@provider
 type mediaAsset struct{}
 
+const (
+	mediaAssetVariantMain    = "main"
+	mediaAssetVariantPreview = "preview"
+)
+
 func mediaAssetTransitionAllowed(from, to consts.MediaAssetStatus) bool {
 	switch from {
 	case consts.MediaAssetStatusUploaded:
@@ -77,6 +82,50 @@ func (s *mediaAsset) AdminUploadInit(ctx context.Context, tenantID, operatorUser
 		return nil, errorx.ErrInvalidParameter.WithMsg("invalid type")
 	}
 
+	variant := strings.TrimSpace(strings.ToLower(form.Variant))
+	if variant == "" {
+		variant = mediaAssetVariantMain
+	}
+	if variant != mediaAssetVariantMain && variant != mediaAssetVariantPreview {
+		return nil, errorx.ErrInvalidParameter.WithMsg("invalid variant")
+	}
+
+	var sourceAssetID int64
+	if form.SourceAssetID != nil {
+		sourceAssetID = *form.SourceAssetID
+	}
+	if variant == mediaAssetVariantMain {
+		if sourceAssetID != 0 {
+			return nil, errorx.ErrInvalidParameter.WithMsg("source_asset_id is only allowed for preview variant")
+		}
+	} else {
+		// preview variant: requires a source main asset for traceability.
+		if sourceAssetID <= 0 {
+			return nil, errorx.ErrInvalidParameter.WithMsg("source_asset_id is required for preview variant")
+		}
+		// 校验来源资源存在、同租户、未删除、且为 main 产物。
+		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, sourceAssetID).
+			Take(&srcRow).Error; err != nil {
+			if errors.Is(err, gorm.ErrRecordNotFound) {
+				return nil, errorx.ErrRecordNotFound.WithMsg("source media asset not found")
+			}
+			return nil, err
+		}
+		srcVariant := srcRow.Variant
+		if srcVariant == "" {
+			srcVariant = mediaAssetVariantMain
+		}
+		if srcVariant != mediaAssetVariantMain {
+			return nil, errorx.ErrPreconditionFailed.WithMsg("source asset must be main variant")
+		}
+	}
+
 	objectKey, err := newObjectKey(tenantID, operatorUserID, typ, now)
 	if err != nil {
 		return nil, pkgerrors.Wrap(err, "generate object_key failed")
@@ -113,11 +162,27 @@ func (s *mediaAsset) AdminUploadInit(ctx context.Context, tenantID, operatorUser
 		return nil, pkgerrors.Wrap(err, "create media asset failed")
 	}
 
+	// variant/source_asset_id 目前为 DB 新增字段；由于 models 为 gen 产物，这里用 SQL 更新列值。
+	updates := map[string]any{
+		"variant": variant,
+	}
+	if sourceAssetID > 0 {
+		updates["source_asset_id"] = sourceAssetID
+	}
+	if err := _db.WithContext(ctx).
+		Model(&models.MediaAsset{}).
+		Where("id = ? AND tenant_id = ?", m.ID, tenantID).
+		Updates(updates).Error; err != nil {
+		return nil, pkgerrors.Wrap(err, "update media asset variant/source_asset_id failed")
+	}
+
 	logrus.WithFields(logrus.Fields{
 		"tenant_id":  tenantID,
 		"user_id":    operatorUserID,
 		"asset_id":   m.ID,
 		"type":       typ,
+		"variant":    variant,
+		"source_id":  sourceAssetID,
 		"object_key": objectKey,
 	}).Info("services.media_asset.admin.upload_init")