nginx worker limit reached on niks3 reverse proxy

```
Jul 06 15:03:34 spike nginx[2130]: 2026/07/06 15:03:34 [alert] 2130#2130: *67889 512 worker_connections are not enough while connecting to upstream, ...
Jul 06 15:03:34 spike nginx[2130]: 2026/07/06 15:03:34 [alert] 2130#2130: 512 worker_connections are not enough
```

resulting in `500 server error` on some requests, seems to be fine because nix will retry pulling from the cache, but this should be fixed

Start by locating the NixOS nginx reverse-proxy configuration for niks3 and inspect the worker connection limit against the logged alerts. Verify the change with requests that exercise the proxy; done means the 500 errors and worker_connections alerts no longer occur under the affected load.

Written by the indexing model from the issue text.