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/ARCHITECTURE.md

@@ -447,3 +447,42 @@ const data = await res.json();
 ### Nota sobre NUMERIC de PostgreSQL
 
 PostgreSQL retorna valores `NUMERIC` como strings. Todas las funciones de formato en JS usan `parseFloat()` para convertir antes de mostrar.
+
+---
+
+## Infraestructura y Monitoreo (2026-04)
+
+### Servicios Systemd
+
+| Servicio | Descripcion | Puerto |
+|----------|-------------|--------|
+| `nexus-pos.service` | Gunicorn POS (Flask) | 5001 |
+| `nexus.service` | Dashboard (Flask) | 5000 |
+| `nexus-quart.service` | Hypercorn async catalog | 5002 |
+| `nexus-celery.service` | Celery workers | — |
+| `nexus-mv-refresh.timer` | Refresh MV diario 03:00 UTC | — |
+| `nexus-cache-warm.timer` | Cache warming diario 04:00 UTC | — |
+
+### Monitoreo (Prometheus + Grafana)
+
+- **Prometheus** :9090 — métricas de sistema, PostgreSQL, Redis
+- **Grafana** :3001 — dashboards visuales (login admin/nexus2026)
+- Exporters: node-exporter, postgres-exporter, redis-exporter
+
+### Stubs de Integraciones de Negocio
+
+- **BNPL** (`bnpl_bp.py`): APLAZO, Kueski, Clip
+- **ERP Sync** (`erp_bp.py`): Aspel SAE, CONTPAQi, SAP B1, Odoo
+- **WhatsApp Meta Cloud** (`whatsapp_cloud_bp.py`): reemplazo escalable de Baileys
+- **Supplier Portal** (`supplier_portal_bp.py`): demanda por zona y top partes
+
+### PWA
+
+- Service Worker (`pos/static/pwa/sw.js`) con cache-first para assets
+- Install prompt (`pos/static/js/pwa-install.js`) captura `beforeinstallprompt`
+- Manifest registrado en 14 templates POS
+
+### Tests E2E
+
+- Playwright + Chromium
+- Primer smoke test: login page loads and rejects invalid credentials