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>

services/network/caddy.nix

@@ -0,0 +1,9 @@
+{ ... }:
+
+# Caddy reverse proxy — base enable + open the web ports.
+# Each host adds its own `services.caddy.virtualHosts.<name>` (LAN names on
+# jupiter, public domains with automatic HTTPS on the neptun).
+{
+  services.caddy.enable = true;
+  networking.firewall.allowedTCPPorts = [ 80 443 ];
+}