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/storage.ts

@@ -53,13 +53,22 @@ export async function putImage(
 }
 
 export async function headImage(key: string): Promise<StoredImage | null> {
-  const indexed = getImageIndexAsStored(key)
-  if (indexed) return indexed
-
   const backend = await getBackendForKey(key)
   const stored = await backend.head(key)
   if (!stored) return null
 
+  const indexed = getImageIndexAsStored(key)
+  if (indexed) {
+    return {
+      ...stored,
+      backendId: backend.id,
+      originalName: indexed.originalName,
+      contentType: indexed.contentType,
+      uploadedAt: indexed.uploadedAt,
+      userId: indexed.userId
+    }
+  }
+
   insertImageIndex({
     key: stored.key,
     backendId: backend.id,