fix(immich): use thumbnail endpoint instead of original for album carousel (#6396)

* fix(immich): use thumbnail endpoint instead of original for album carousel

Switches from /api/assets/:id/original to /api/assets/:id/thumbnail to fix
HEIC image incompatibility (thumbnails are always JPEG), reduce bandwidth,
and improve loading speed.

Closes #6388

* working on immich

* Improve image proxy error handling

* fix(image-proxy): use trusted fetch response type

* docs(immich): require Asset View permission

apps/nextjs/src/app/api/image-proxy/[id]/route.ts

@@ -1,19 +1,25 @@
-import { notFound } from "next/navigation";
-
 import { ImageProxy } from "@homarr/image-proxy";
 
 export const GET = async (_request: Request, props: { params: Promise<{ id: string }> }) => {
   const { id } = await props.params;
 
   const imageProxy = new ImageProxy();
-  const image = await imageProxy.forwardImageAsync(id);
-  if (!image) {
-    notFound();
+  const result = await imageProxy.forwardImageAsync(id);
+
+  if ("error" in result) {
+    const status = result.error === "upstream-error" ? result.statusCode : result.error === "fetch-error" ? 502 : 404;
+    return new Response(null, {
+      status,
+      headers: {
+        "X-Homarr-Image-Proxy-Error": result.error,
+      },
+    });
   }
 
-  return new Response(image, {
+  return new Response(result.image, {
     headers: {
       "Cache-Control": "public, max-age=3600, immutable", // Cache for 1 hour
+      ...(result.contentType ? { "Content-Type": result.contentType } : {}),
     },
   });
 };