feat(backend): update payments handlers and service

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/payments/validators.go

@@ -58,4 +58,4 @@ func ValidateCardInfo(cardID, newCardToken *string) error {
 		return errors.New("either card_id or new_card_token is required")
 	}
 	return nil
-}
+}