feat(attic): add daily garbage collection timer

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

systems/x86_64-linux/attic/atticd.nix

@@ -61,4 +61,25 @@
       };
     };
   };
+
+  systemd.services.atticd-gc = {
+    description = "Attic garbage collection";
+    requires = [ "atticd.service" ];
+    after = [ "atticd.service" ];
+    serviceConfig = {
+      Type = "oneshot";
+      ExecStart = "${pkgs.attic-server}/bin/atticd-atticadm gc";
+      EnvironmentFile = "/etc/atticd.env";
+    };
+  };
+
+  systemd.timers.atticd-gc = {
+    description = "Daily Attic garbage collection";
+    wantedBy = [ "timers.target" ];
+    timerConfig = {
+      OnCalendar = "daily";
+      Persistent = true;
+      RandomizedDelaySec = "1h";
+    };
+  };
 }