feat: MercadoLibre integration + inventory bulk publish + WhatsApp bridge fixes

- Add MercadoLibre OAuth, listings, orders, webhooks and category search
- New marketplace_external_bp.py, meli_service.py, marketplace_external_service.py
- New marketplace_external.html/js with ML management UI
- Inventory: bulk publish to ML with category autocomplete, listing type and shipping selectors
- Inventory: new .btn--meli styles, select/label CSS fixes
- WhatsApp bridge: rate limiting, 440/515/408 error handling, stale watchdog
- DB migration v3.4_meli_integration.sql for marketplace_listings, orders, sync_queue
- Add Celery tasks for ML sync and webhook processing
- Sidebar: MercadoLibre navigation link

pos/templates/customers.html

@@ -420,6 +420,10 @@
                     <div class="credit-metric__label">Utilizado</div>
                     <div class="credit-metric__value used" id="detailCreditUsed">$31,500</div>
                   </div>
+                  <div class="credit-metric">
+                    <div class="credit-metric__label">Descuento Max</div>
+                    <div class="credit-metric__value" id="detailMaxDiscount">0%</div>
+                  </div>
                 </div>
                 <div class="credit-progress">
                   <div class="credit-progress__labels">
@@ -611,7 +615,7 @@
     </div>
     <div class="panel-footer">
       <button class="btn-edit" onclick="if(typeof Customers!=='undefined') Customers.editCurrent();">Editar Cliente</button>
-      <button class="btn-sale" onclick="window.location.href='/pos/';">Nueva Venta</button>
+      <button class="btn-sale" onclick="window.location.href='/pos/sale';">Nueva Venta</button>
     </div>
   </div>