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/database/migrations/20251222175500_media_assets_source_asset_id.sql

@@ -0,0 +1,16 @@
+-- +goose Up
+-- +goose StatementBegin
+ALTER TABLE media_assets
+    ADD COLUMN IF NOT EXISTS source_asset_id bigint;
+
+COMMENT ON COLUMN media_assets.source_asset_id IS '派生来源资源ID：preview 产物可指向对应 main 资源；用于建立 preview/main 的 1:1 追溯关系';
+
+CREATE INDEX IF NOT EXISTS ix_media_assets_tenant_source_asset_id ON media_assets (tenant_id, source_asset_id);
+-- +goose StatementEnd
+
+-- +goose Down
+-- +goose StatementBegin
+DROP INDEX IF EXISTS ix_media_assets_tenant_source_asset_id;
+ALTER TABLE media_assets DROP COLUMN IF EXISTS source_asset_id;
+-- +goose StatementEnd
+