fix: handle Pydantic v2 model serialization in save_subscriptions method

ss/storage.py

@@ -53,6 +53,10 @@ class StorageManager:
         try:
             with open(self.subscriptions_file, 'w', encoding='utf-8') as f:
                 # Convert Pydantic model to dict for YAML serialization
+                if hasattr(subscriptions, 'model_dump'):
+                    data = subscriptions.model_dump(mode='json')
+                else:
+                    # Fallback for Pydantic v1
                     data = subscriptions.dict()
                 yaml.dump(data, f, default_flow_style=False, allow_unicode=True)
             return True