paperless: enforce data directory presence

In the case of mounted directories, systemd needs to know that this
unit depends on this path to add the adequate dependency.

(Technically: these units, but adding one dependency is sufficient).

nixos/modules/services/misc/paperless.nix

@@ -485,6 +485,7 @@ in
             PrivateNetwork = cfg.database.createLocally; # defaultServiceConfig enables this by default, needs to be disabled for remote DBs
           };
           environment = env;
+          unitConfig.RequiresMountsFor = defaultServiceConfig.ReadWritePaths;
 
           preStart = ''
               # remove old papaerless-manage symlink