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/blueprints/customers_bp.py

@@ -91,7 +91,7 @@ def get_customer(customer_id):
     cur.execute("""
         SELECT id, branch_id, name, rfc, razon_social, regimen_fiscal, uso_cfdi,
                cp, email, phone, address, price_tier, credit_limit, credit_balance,
-               is_active, vehicle_info, created_at
+               is_active, vehicle_info, created_at, max_discount_pct
         FROM customers WHERE id = %s
     """, (customer_id,))
     row = cur.fetchone()
@@ -103,7 +103,7 @@ def get_customer(customer_id):
     customer = dict(zip(cols, row))
 
     # Convert Decimal to float
-    for k in ('credit_limit', 'credit_balance'):
+    for k in ('credit_limit', 'credit_balance', 'max_discount_pct'):
         if customer.get(k) is not None:
             customer[k] = float(customer[k])
 
@@ -213,7 +213,7 @@ def update_customer(customer_id):
     # Build dynamic update
     allowed = ['name', 'rfc', 'razon_social', 'regimen_fiscal', 'uso_cfdi',
                'cp', 'email', 'phone', 'address', 'price_tier', 'credit_limit',
-               'vehicle_info', 'is_active', 'branch_id']
+               'max_discount_pct', 'vehicle_info', 'is_active', 'branch_id']
     sets = []
     vals = []
     for field in allowed: