nixos/paperless: add system-paperless.slice

Non-breaking change.

Part of #279915.

nixos/modules/services/misc/paperless.nix

@@ -40,8 +40,9 @@ let
     exec ${cfg.package}/bin/paperless-ngx "$@"
   '';
 
-  # Secure the services
   defaultServiceConfig = {
+    Slice = "system-paperless.slice";
+    # Secure the services
     ReadWritePaths = [
       cfg.consumptionDir
       cfg.dataDir
@@ -232,6 +233,11 @@ in
   config = mkIf cfg.enable {
     services.redis.servers.paperless.enable = mkIf enableRedis true;
 
+    systemd.slices.system-paperless = {
+      description = "Paperless slice";
+      documentation = [ "https://paperless.readthedocs.io" ];
+    };
+
     systemd.tmpfiles.settings."10-paperless" = let
       defaultRule = {
         inherit (cfg) user;