docs: update API, architecture, installation guides and README

- API-POS.md: added sections 11-15 (BNPL, ERP, WhatsApp Cloud,
  Supplier Portal, Dashboard Stats)
- ARCHITECTURE.md: added infra/monitoring section with systemd,
  Prometheus/Grafana, PWA, and integration stubs
- INSTALACION.md: added systemd setup, monitoring docker compose,
  PWA install notes, and Playwright test commands
- README.md: updated endpoint count, tech stack, infrastructure table

docs/INSTALACION.md

@@ -280,3 +280,39 @@ sudo -u postgres psql tenant_mi_refaccionaria -c "SELECT name, role FROM employe
 | "Tenant not found" | Verificar que `provision_tenant` se ejecuto correctamente |
 | PIN bloqueado | Esperar 15 minutos o reiniciar el servicio POS |
 | Base de datos no conecta | Verificar credenciales en `pos/config.py` |
+
+---
+
+## Servicios Systemd (Produccion)
+
+Despues de la instalacion, copiar los servicios y habilitarlos:
+
+```bash
+sudo cp systemd/*.service systemd/*.timer /etc/systemd/system/
+sudo systemctl daemon-reload
+sudo systemctl enable --now nexus-pos.service nexus.service nexus-celery.service nexus-quart.service
+sudo systemctl enable --now nexus-mv-refresh.timer nexus-cache-warm.timer
+```
+
+## Monitoreo (Opcional)
+
+Levantar Prometheus + Grafana:
+
+```bash
+cd docker
+docker compose -f docker-compose.monitoring.yml up -d
+```
+
+- Grafana: http://servidor:3001 (admin / nexus2026)
+- Prometheus: http://servidor:9090
+
+## PWA (Instalable)
+
+El POS ya incluye Service Worker y manifest. Al abrir cualquier pagina del POS en Chrome/Edge, aparecera un banner para instalar la app. Requiere HTTPS en produccion para el prompt automatico.
+
+## Tests E2E (Playwright)
+
+```bash
+npm install
+npx playwright test
+```