feat(publish): require cover image for WeChat Official Account

WeChat 公众号在管理员/运营者授权发布场景下需要 2.35:1 封面，
新增共享 publishPlatformRequiresCover 校验并同步前端封面要求与文案，
避免在调度与即时发布两条路径上重复维护平台名单。

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

server/internal/tenant/app/schedule_task_service.go

@@ -564,8 +564,7 @@ func (s *ScheduleTaskService) loadPublishAccountPlatformIDs(ctx context.Context,
 
 func schedulePublishPlatformsRequireCover(platformIDs []string) bool {
 	for _, platformID := range platformIDs {
-		switch strings.TrimSpace(platformID) {
-		case "baijiahao", "dongchedi":
+		if publishPlatformRequiresCover(platformID) {
 			return true
 		}
 	}