ensure ddclient service is running; default protocols to []

- Add an explicit service state=started enabled=true task so the role is self-healing if the unit is disabled or stopped, rather than relying solely on the restart handler.
- Default ddclient_protocols to [] so the template's for-loop no longer errors when the var is omitted.
- README: note service management in the tagline and document the new ddclient_protocols default.

tasks/main.yaml

@@ -19,3 +19,9 @@
     owner: root
     mode: "0600"
   notify: Restart ddclient service
+
+- name: Ensure ddclient service is enabled and running
+  ansible.builtin.service:
+    name: ddclient
+    state: started
+    enabled: true