Reorganize services/ into category subfolders

Group service modules by category (media, network, vpn, identity,
dev, desktop) to make the growing services/ dir easier to navigate.
containers.nix stays at the top level since it's a shared backend,
not a single-category service.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

flake.nix

@@ -73,8 +73,8 @@
           modules = [
             (nixpkgs + "/nixos/modules/virtualisation/qemu-vm.nix")
             ./common.nix
-            ./services/unbound.nix
-            ./services/pihole.nix
+            ./services/network/unbound.nix
+            ./services/network/pihole.nix
             ({ lib, ... }: {
               networking.hostName = "mercury-vm";
               networking.nameservers = [ "1.1.1.1" "9.9.9.9" ];   # host resolver (not pihole)