fix(audit): corrige errores criticos y mayores, mejora UX/accesibilidad y optimiza rendimiento

- Arregla @require_auth, permisos, race conditions, locks de caja/stock
- Elimina N+1 en layaway, flotilla, dashboard y global_invoice
- Asegura folios atomicos para CFDI, ordenes de servicio y polizas
- Protege client_secret de MercadoLibre en backend
- Conecta botones/filtros de config, customers, accounting e invoicing
- Mejora accesibilidad (labels/aria-label) y estados de carga/vacio
- Limpia accounting.js obsoleto y consolida accounting.v9.js
- Actualiza cache busting a v32 y Service Worker a v32
- Documenta todo en docs/AUDIT_Y_MEJORAS_2026-06-15.md

Tests: 35 passed

pos/migrations/v4.8_workshop_permissions.sql

@@ -0,0 +1,15 @@
+-- v4.8 Workshop permissions seed
+-- Grants workshop permissions to existing admin employees so they keep access
+-- after the new role-based restrictions are enforced.
+
+INSERT INTO employee_permissions (employee_id, permission)
+SELECT id, 'workshop.view'
+FROM employees
+WHERE role = 'admin'
+ON CONFLICT DO NOTHING;
+
+INSERT INTO employee_permissions (employee_id, permission)
+SELECT id, 'workshop.edit'
+FROM employees
+WHERE role = 'admin'
+ON CONFLICT DO NOTHING;