nginx: move 80.conf to sites-available so it's actually included

The bundle was shipping 80.conf (HTTP-to-HTTPS redirect + acme-challenge
alias) to /etc/nginx/sites/80.conf, but nginx.conf only `include`s
/etc/nginx/sites-enabled/* (which is a symlink to sites-available).
The file was orphaned — no node had a working port-80 listener.

Move the destination to /etc/nginx/sites-available/80.conf so the
existing sites-enabled symlink picks it up. The /etc/nginx purge will
clean up any stale /etc/nginx/sites/80.conf on existing hosts.

bundles/nginx/items.py

@@ -64,7 +64,7 @@ files = {
             'svc_systemd:nginx:restart',
         },
     },
-    '/etc/nginx/sites/80.conf': {
+    '/etc/nginx/sites-available/80.conf': {
         'triggers': {
             'svc_systemd:nginx:restart',
         },