refactor: split services into reusable services/ modules

- services/{samba,avahi,audiobookshelf,containers,caddy,tailscale}.nix
- common.nix grows firewall base + timezone; hosts import what they need
- jupiter/vm/vps import service modules; drop the jupiter/services.nix monolith
- each module opens its own firewall ports; caddy/tailscale shared by hosts
- verified: jupiter/vps/vbox eval + jupiter builds, config equivalent

services/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 vps).
+{
+  services.caddy.enable = true;
+  networking.firewall.allowedTCPPorts = [ 80 443 ];
+}