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>

tests/test_ddns_endpoints.py

@@ -136,5 +136,38 @@ class TestUpdateDdnsConfig(unittest.TestCase):
         mock_id.assert_any_call('domain_name', 'home.example.com')
 
 
+class TestDdnsRegister(unittest.TestCase):
+
+    def setUp(self):
+        self.client = _make_client()
+
+    def test_non_pic_ngo_provider_returns_400(self):
+        from app import config_manager
+        with patch.object(config_manager, 'configs', {'ddns': {'provider': 'cloudflare'}, '_identity': {}}):
+            r = self.client.post('/api/ddns/register')
+        self.assertEqual(r.status_code, 400)
+
+    def test_missing_cell_name_returns_400(self):
+        from app import config_manager
+        with patch.object(config_manager, 'configs', {'ddns': {'provider': 'pic_ngo'}, '_identity': {}}):
+            r = self.client.post('/api/ddns/register')
+        self.assertEqual(r.status_code, 400)
+        self.assertIn('cell_name', json.loads(r.data)['error'])
+
+    def test_register_success(self):
+        from app import config_manager
+        from ddns_manager import DDNSManager
+        with patch.object(config_manager, 'configs', {
+            'ddns': {'provider': 'pic_ngo'},
+            '_identity': {'cell_name': 'mypic'}
+        }):
+            with patch.object(DDNSManager, 'register', return_value={'subdomain': 'mypic.pic.ngo', 'token': 'tok'}) as mock_reg, \
+                 patch.object(config_manager, 'set_identity_field') as mock_id:
+                r = self.client.post('/api/ddns/register')
+        self.assertEqual(r.status_code, 200)
+        body = json.loads(r.data)
+        self.assertTrue(body['registered'])
+        self.assertEqual(body['subdomain'], 'mypic.pic.ngo')
+
 if __name__ == '__main__':
     unittest.main()