xmpp_cert_sync: renew the XMPP certificate through Traefik

The ejabberd certificate (asxp.io, conference., xmpp.) was issued by
hand and expires 2027-01-28 with nothing renewing it. Traefik now
obtains it alongside the sites' certificates, through a router that
only exists to request it, and a systemd path unit on ops pushes it to
services whenever acme.json changes, with a daily run as fallback.

The push uses its own SSH key, accepted from ops' VPC address only and
forced to xmpp-cert-install, which checks the certificate is valid for
another day and matches its key before replacing server.pem and
reloading ejabberd.

Two alerts: the sync hasn't succeeded for two days, or ejabberd serves
an older certificate than Traefik holds for six hours. The existing
expiry alert already covers port 5223.

Assisted-by: Claude:opus-5-5

ansible/roles/xmpp_cert_sync/handlers/main.yaml

@@ -0,0 +1,4 @@
+---
+- name: Reload systemd
+  ansible.builtin.systemd:
+    daemon_reload: true