feat(pm/umber): init grocy

Grocy is a stock tracking application for groceries. I'd like to use it
to keep track of some of my stuff, so let's host it on umber!

packetmix/systems/umber/grocy.nix

···
+# SPDX-FileCopyrightText: 2025 FreshlyBakedCake
+#
+# SPDX-License-Identifier: MIT
+
+{ lib, ... }: {
+  services.grocy = {
+    enable = true;
+    hostName = "grocy.starrysky.fyi";
+
+    settings.currency = "GBP";
+  };
+
+  services.nginx.virtualHosts."grocy.starrysky.fyi" = {
+    acmeRoot = null;
+    forceSSL = lib.mkForce false;
+    onlySSL = true;
+  };
+
+  clicks.storage.impermanence.persist.directories = [ "/var/lib/grocy" ];
+}