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/media_service.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"crypto/rand"
 	"encoding/hex"
+	"strings"
 	"time"
 
 	"github.com/jackc/pgx/v5/pgxpool"
@@ -136,13 +137,22 @@ func (s *MediaService) listArticlePublishRecords(ctx context.Context, tenantID,
 
 func publishBatchRequiresCover(accounts []platformAccountSeed) bool {
 	for _, account := range accounts {
-		if account.PlatformID == "baijiahao" || account.PlatformID == "dongchedi" {
+		if publishPlatformRequiresCover(account.PlatformID) {
 			return true
 		}
 	}
 	return false
 }
 
+func publishPlatformRequiresCover(platformID string) bool {
+	switch strings.TrimSpace(platformID) {
+	case "weixin_gzh", "baijiahao", "dongchedi":
+		return true
+	default:
+		return false
+	}
+}
+
 func pointerStringValue(value *string) string {
 	if value == nil {
 		return ""