feat: MercadoLibre mejoras - importar existentes, sync stock, sync ordenes

- meli_service.py: agrega get_user_items() para obtener publicaciones del vendedor
- marketplace_external_service.py:
  - import_existing_listings(): importa publicaciones existentes de ML a marketplace_listings
  - process_meli_sync_queue(): procesa cola de sincronizacion de stock a ML
  - Actualiza stock en ML via update_item(available_quantity)
- marketplace_external_bp.py:
  - POST /listings/import-existing - importa publicaciones existentes
  - POST /sync-stock - procesa cola de stock manualmente
  - POST /orders/sync - sincroniza ordenes manualmente
- inventory_engine.py: inserta en meli_sync_queue tras cada operacion de inventario
- migration v4.2: crea tabla meli_sync_queue

Prueba en tenant_refaccionaria_rached: 52 publicaciones importadas exitosamente

pos/services/inventory_engine.py

@@ -121,6 +121,18 @@ def record_operation(conn, inventory_id, branch_id, operation_type, quantity,
         notes
     ))
     op_id = cur.fetchone()[0]
+
+    # Queue ML stock sync if this product has an active ML listing
+    cur.execute("""
+        INSERT INTO meli_sync_queue (inventory_id, action, status)
+        SELECT %s, 'stock_update', 'pending'
+        WHERE EXISTS (
+            SELECT 1 FROM marketplace_listings
+            WHERE inventory_id = %s AND channel = 'mercadolibre' AND is_active = true
+        )
+        ON CONFLICT DO NOTHING
+    """, (inventory_id, inventory_id))
+
     cur.close()
     return op_id