feat(media): require cover image for dongchedi publish batches

懂车帝 has the same cover-mandatory rule as 百家号; treat them the same
when deciding whether a publish batch must carry a cover.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/internal/tenant/app/media_service.go

@@ -136,7 +136,7 @@ func (s *MediaService) listArticlePublishRecords(ctx context.Context, tenantID,
 
 func publishBatchRequiresCover(accounts []platformAccountSeed) bool {
 	for _, account := range accounts {
-		if account.PlatformID == "baijiahao" {
+		if account.PlatformID == "baijiahao" || account.PlatformID == "dongchedi" {
 			return true
 		}
 	}