perf: skip Next.js image re-optimization and stream proxy responses

Thumbnails are already optimized (WebP) by the API, so disable Next.js
image optimization to avoid redundant CPU work. Switch route handlers
from buffering (arrayBuffer) to streaming (response.body) to reduce
memory usage and latency.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/backoffice/next.config.mjs

@@ -4,6 +4,7 @@ const nextConfig = {
   typedRoutes: true,
   images: {
     minimumCacheTTL: 86400,
+    unoptimized: true,
   },
 };