fix: resolve 4 media embed bugs from testing

1. Video aspect ratio: remove container border/overflow-hidden, use
   preload="metadata" so browser knows dimensions before play
2. YouTube Error 153: remove sandbox attr (too restrictive), add full
   allow permissions (encrypted-media, accelerometer, gyroscope, etc.)
3. Google Images not displaying: detect image Content-Type from HTTP
   response in metadataFetcher, override classifier to create image
   embed for URLs that serve image/* content
4. Audio seeking broken: add HTTP Range request support in uploads
   route (Accept-Ranges, Content-Range, 206 Partial Content)

packages/web/src/components/chat/AttachmentRenderer.tsx

@@ -44,10 +44,10 @@ export function AttachmentRenderer({ attachment }: AttachmentRendererProps) {
 
   if (mimetype.startsWith('video/')) {
     return (
-      <div className="mt-1 max-w-[520px] rounded-lg overflow-hidden border border-white/[0.06]">
+      <div className="mt-1 max-w-[520px]">
         <video
           controls
-          preload="none"
+          preload="metadata"
           poster={thumbUrl ?? undefined}
           className="max-w-full max-h-[400px] rounded-lg"
         >