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/pages/Settings.jsx

@@ -444,6 +444,7 @@ function Settings() {
   const [ddnsDuckStatus, setDdnsDuckStatus] = useState(null);
   const [ddnsDirty, setDdnsDirty] = useState(false);
   const [ddnsSaving, setDdnsSaving] = useState(false);
+  const [ddnsRegistering, setDdnsRegistering] = useState(false);
 
   // service configs
   const [serviceConfigs, setServiceConfigs] = useState({});
@@ -595,6 +596,21 @@ function Settings() {
     }
   }, [ddnsCfToken, domainName]);
 
+  const reRegister = useCallback(async () => {
+    setDdnsRegistering(true);
+    try {
+      const res = await ddnsAPI.register();
+      setDomainName(res.data.subdomain || '');
+      setDdnsHasToken(true);
+      setPicAvail(null);
+      toast(`Registered as ${res.data.subdomain}`);
+    } catch (err) {
+      toast(err.response?.data?.error || 'Registration failed', 'error');
+    } finally {
+      setDdnsRegistering(false);
+    }
+  }, []);
+
   const verifyDuck = useCallback(async () => {
     if (!ddnsDuckToken.trim()) return;
     setDdnsDuckStatus('checking');
@@ -865,9 +881,18 @@ function Settings() {
       <Section icon={Globe} title="External Domain & DDNS" collapsible defaultOpen>
         <div className="space-y-3">
           {domainMode === 'pic_ngo' && (
-            <div className="rounded-lg bg-blue-50 border border-blue-100 px-4 py-3 text-sm text-blue-700">
-              Your cell is registered as <span className="font-mono font-semibold">{domainName || `${identity.cell_name}.pic.ngo`}</span> on pic.ngo.
-              Change the Cell Name above to update this subdomain.
+            <div className="space-y-2">
+              <div className="rounded-lg bg-blue-50 border border-blue-100 px-4 py-3 text-sm text-blue-700">
+                Your cell is registered as <span className="font-mono font-semibold">{domainName || `${identity.cell_name}.pic.ngo`}</span> on pic.ngo.
+                Change the Cell Name above to update this subdomain.
+              </div>
+              <button
+                onClick={reRegister}
+                disabled={ddnsRegistering}
+                className="px-3 py-1.5 text-xs font-medium rounded border border-blue-300 text-blue-700 hover:bg-blue-50 disabled:opacity-50"
+              >
+                {ddnsRegistering ? 'Registering…' : 'Re-register with pic.ngo'}
+              </button>
             </div>
           )}
           {domainMode === 'cloudflare' && (