feat: DDNS self-healing heartbeat + manual re-register endpoint

- DDNSTokenExpired exception triggers auto re-register in update_ip()
  so cells recover silently after a DDNS DB reset
- POST /api/ddns/register lets the user force re-registration from Settings
- Re-register button in Settings → External Domain & DDNS (pic_ngo only)
- 3 new tests covering register endpoint: wrong provider, missing name, success

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

webui/src/services/api.js

@@ -325,6 +325,7 @@ export const logsAPI = {
 export const ddnsAPI = {
   checkName: (name) => api.get(`/api/ddns/check/${name}`),
   updateConfig: (data) => api.put('/api/ddns', data),
+  register: () => api.post('/api/ddns/register'),
 };
 
 // Setup Wizard API