paperless: fix uploading files via local folder

The consumer service needs to be in the same namespace as the
task-queue, otherwise they cannot see each others temporary files.
Same as for web uploading.

See also the discussion here:
https://github.com/paperless-ngx/paperless-ngx/discussions/5606

nixos/modules/services/misc/paperless.nix

@@ -307,6 +307,9 @@ in
         Restart = "on-failure";
       };
       environment = env;
+      # Allow the consumer to access the private /tmp directory of the server.
+      # This is required to support consuming files via a local folder.
+      unitConfig.JoinsNamespaceOf = "paperless-task-queue.service";
     };
 
     systemd.services.paperless-web = {