caddy: add a [caddy] role group and target it

services/caddy_playbook.yml was the one play still targeting a location
group (vps) rather than a role group. The two coincide today — vps is
exactly vipy, watchtower and spacey, the three hosts with
/etc/caddy/sites-enabled — but adding a fourth VPS that does not run
Caddy would have silently pulled it into the play.

[caddy:children] is edge + monitoring + vpn_control. Verified the play
selects the same three machines before and after.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ansible/inventory.ini

@@ -59,3 +59,9 @@ localhost
 vps
 nodito_host
 nodito_vms
+
+# Hosts that run Caddy and therefore have /etc/caddy/sites-enabled.
+[caddy:children]
+edge
+monitoring
+vpn_control

ansible/services/caddy_playbook.yml

@@ -1,5 +1,5 @@
 - name: Install and configure Caddy on Debian 12
-  hosts: vps
+  hosts: caddy
   become: yes
 
   tasks: