Add gzip_proxied any to enable compression behind reverse proxies

GCP HTTP(S) LB adds Via header to forwarded requests, causing nginx to
treat them as proxied. Without gzip_proxied, nginx skips dynamic gzip
for these requests, resulting in uncompressed JS/CSS responses.

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

flake.nix

@@ -55,6 +55,7 @@
                 access_log /dev/stdout;
                 server {
                   gzip on;
+                  gzip_proxied any;
                   gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;
                   listen 80;
                   index index.php index.html;