Stream video uploads to disk instead of buffering in memory

req.formData() parsed the entire multipart body into RAM, so large
uploads (1.6GB+) OOM-killed the server. Bridge the web Request body to
a Node stream and parse it with formidable, which writes the file to
disk as it arrives:

- Temp dir is /uploads/tmp (same filesystem as the destination) so the
  post-parse move is an atomic rename, not a second multi-GB write
- Mark the bridged request as chunked when a proxy strips
  content-length, otherwise formidable assumes an empty body
- Track formidable temp files via fileBegin so aborted uploads get
  their partial files cleaned up
- Remove the dead formidable branch gated on (req as any).req, which
  is always null in the App Router

Also fix the nginx example in VPS_UPLOAD_CONFIG.md: client_max_body_size
was 1024M, below the app's 2.5GB limit, and would 413 large uploads at
the proxy before they reached the app.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

VPS_UPLOAD_CONFIG.md

@@ -63,9 +63,10 @@ If running with a proxy (nginx/Apache), ensure:
 location /api/admin/upload {
     proxy_pass http://next-server;
     proxy_connect_timeout 60s;
-    proxy_send_timeout 300s;        # 5 minutes for large uploads
-    proxy_read_timeout 300s;        # 5 minutes for large uploads
-    client_max_body_size 1024M;     # Adjust based on your max video size
+    proxy_send_timeout 2700s;       # 45 minutes, matches maxDuration in the route
+    proxy_read_timeout 2700s;       # 45 minutes, matches maxDuration in the route
+    client_max_body_size 3G;        # Must exceed the 2.5GB app-level limit
+    proxy_request_buffering off;    # Stream to the app instead of spooling to nginx disk
 }
 ```