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>

api/routes/config.py

@@ -561,6 +561,30 @@ def update_ddns_config():
         return jsonify({'error': str(e)}), 500
 
 
+@bp.route('/api/ddns/register', methods=['POST'])
+def ddns_register():
+    """Trigger (re-)registration with the configured DDNS provider."""
+    try:
+        from app import config_manager
+        ddns_cfg = config_manager.configs.get('ddns', {})
+        if ddns_cfg.get('provider') != 'pic_ngo':
+            return jsonify({'error': 'Re-registration only supported for pic_ngo provider'}), 400
+        identity = config_manager.configs.get('_identity', {})
+        cell_name = identity.get('cell_name', os.environ.get('CELL_NAME', ''))
+        if not cell_name:
+            return jsonify({'error': 'cell_name not configured'}), 400
+        from ddns_manager import DDNSManager as _DDNSManager
+        _mgr = _DDNSManager(config_manager)
+        result = _mgr.register(cell_name, '')
+        new_sub = result.get('subdomain', f'{cell_name}.pic.ngo')
+        config_manager.set_identity_field('domain_name', new_sub)
+        logger.info('DDNS registered via /api/ddns/register: cell_name=%r subdomain=%r', cell_name, new_sub)
+        return jsonify({'registered': True, 'subdomain': new_sub})
+    except Exception as e:
+        logger.error('Error in /api/ddns/register: %s', e)
+        return jsonify({'error': str(e)}), 500
+
+
 @bp.route('/api/config/pending', methods=['GET'])
 def get_pending_config():
     from app import config_manager