BERLIN (Reuters) - Chancellor Angela Merkel's authority within her center-right coalition was at stake on Thursday when uneasy German lawmakers vote on Berlin's contribution to a euro zone aid package for Spain's ailing banks. Merkel can count on broad center-left opposition support to push the decision through the Bundestag, the lower house of parliament, whose members were recalled to rainy Berlin from their summer recess especially for this one-day session. ...