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/middleware/00-host-isolation.ts

@@ -1,7 +1,10 @@
 import { shouldBlockImageHostRequest, shouldBlockSiteHostImageRequest } from '../utils/host-isolation'
 
+const HOST_ISOLATION_CACHE_CONTROL = 'private, no-cache, max-age=0'
+
 export default defineEventHandler((event) => {
   if (shouldBlockImageHostRequest(event) || shouldBlockSiteHostImageRequest(event)) {
+    setHeader(event, 'Cache-Control', HOST_ISOLATION_CACHE_CONTROL)
     throw createError({ statusCode: 404, statusMessage: 'Not Found' })
   }
 })