hotfix: corrige 500 en /customers y /historical-sales

- customers_bp: last_purchase no existe como columna; se calcula via subquery a sales.
- pos_bp: historical_sales no existe en tenants sin importacion; endpoint devuelve lista vacia en lugar de 500.
- Servicio reiniciado y validado.

Tests: 35 passed

pos/blueprints/customers_bp.py

@@ -80,7 +80,8 @@ def list_customers():
         SELECT c.id, c.name, c.rfc, c.razon_social, c.phone, c.email,
                c.address, c.cp,
                c.price_tier, c.credit_limit, c.credit_balance, c.vehicle_info,
-               c.branch_id, c.is_active, c.created_at, c.last_purchase
+               c.branch_id, c.is_active, c.created_at,
+               (SELECT MAX(s.created_at) FROM sales s WHERE s.customer_id = c.id) AS last_purchase
         FROM customers c
         WHERE {where}
         ORDER BY c.name