fix: gate ZNC secret on service being enabled to fix zeta activation

The agenix secret for ZNC was unconditionally setting owner/group to
znc:znc, but with services.znc.enable = false the znc user doesn't
exist, causing activation to fail with "chown: invalid user: 'znc:znc'".

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

Author: stackptr

hosts/zeta/services/znc.nix

@@ -1,9 +1,10 @@
 {
   config,
+  lib,
   pkgs,
   ...
 }: {
-  age.secrets.znc-conf = {
+  age.secrets.znc-conf = lib.mkIf config.services.znc.enable {
     file = ./../secrets/znc-conf.age;
     owner = config.services.znc.user;
     inherit (config.services.znc) group;