fix: nginx syslog access_log tag must not contain a hyphen

The nginx_access syslog tag added in #37 (feat/emily-incident-logging)
used tag=nginx-access. nginx's syslog sink only accepts alphanumeric
characters and underscores in the tag field, so the generated
nginx.conf failed its config test on linode:

  nginx: [emerg] syslog "tag" only allows alphanumeric characters
  and underscore in .../nginx.conf:114

Because nginx-pre-start failed, nginx.service crash-looped until it
hit systemd's start-limit-hit and gave up entirely. Since Nextcloud is
proxied through nginx (127.0.0.1:8080, fronted by haproxy's 'next'
backend), this took next.thehellings.com down with a 503 from haproxy
(phpfpm-nextcloud/postgresql/redis backends were all healthy and
unaffected - purely an nginx config parse failure).

Fix: use an underscore (nginx_access) instead of a hyphen.

hosts/unstable/linode/default.nix

@@ -325,10 +325,19 @@ in
 
       # Route nginx access logs through syslog/journald (rather than only to
       # /var/log/nginx/access.log, which the read-only monitoring account
-      # can't read) so `journalctl -t nginx-access` gives visibility into
+      # can't read) so `journalctl -t nginx_access` gives visibility into
       # Nextcloud request traffic during bandwidth investigations.
+      #
+      # NOTE: nginx's syslog "tag" only allows alphanumeric characters and
+      # underscores (no hyphens) - an earlier version of this used
+      # tag=nginx-access, which fails nginx's config test with:
+      #   nginx: [emerg] syslog "tag" only allows alphanumeric characters
+      #   and underscore in .../nginx.conf:114
+      # That broke nginx.service (and, transitively, Nextcloud/next.thehellings.com,
+      # which is proxied through nginx on 127.0.0.1:8080) until nginx hit its
+      # systemd restart limit and gave up (start-limit-hit).
       appendHttpConfig = ''
-        access_log syslog:server=unix:/dev/log,tag=nginx-access combined;
+        access_log syslog:server=unix:/dev/log,tag=nginx_access combined;
       '';
     };