fix: gallery preview retries and avoid caching failed image responses

Verify storage files before serving from the index, disable CDN caching
on image 404/403 and host-isolation blocks, and retry gallery thumbnails
after transient load failures.

server/utils/serve-image.ts

@@ -20,7 +20,7 @@ import { getBackendRowForKey } from './storage/resolver'
  * - Referer 可被非浏览器客户端伪造，本方案防普通网站盗链，不是身份认证
  */
 
-const ERROR_CACHE_CONTROL = 'public, max-age=60'
+const ERROR_CACHE_CONTROL = 'private, no-cache, max-age=0'
 
 function getAllowedHosts(event: H3Event): Set<string> {
   const hosts = new Set<string>()