fix: COS 自定义域名空值时回退到原生域名

- CosStorageProvider 初始化时 Domain 为空不再传入 SDK
- 原生 COS 域名 (cos.ap-beijing.myqcloud.com) 有有效 TLS 证书
- 生产环境移除 STORAGE_COS_DOMAIN 配置

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/infrastructure/storage/cos-storage.provider.ts

@@ -33,7 +33,7 @@ export class CosStorageProvider {
     this.cos = new COS({
       SecretId: secretId,
       SecretKey: secretKey,
-      Domain: domain,
+      ...(domain ? { Domain: domain } : {}),
       Protocol: 'https:',
     });