docs(infra): add PostgreSQL tuning and systemd service documentation

- POSTGRESQL_TUNING.md: documents applied config (8GB shared_buffers,
  64MB work_mem, 8GB max_wal_size, SSD params)
- SYSTEMD_SERVICES.md: lists all production systemd services
- systemd/: versioned copies of all .service and .timer files
- .gitignore: ignore package-lock.json and backups/

docs/SYSTEMD_SERVICES.md

@@ -0,0 +1,35 @@
+# Systemd Services — Nexus Autoparts
+
+All production services are managed via systemd. Files are versioned in `systemd/`.
+
+## Services
+
+| Service | Description | Status |
+|---------|-------------|--------|
+| `nexus-pos.service` | Gunicorn POS (Flask), port 5001 | Active |
+| `nexus.service` | Dashboard (Flask), port 5000 | Active |
+| `nexus-quart.service` | Hypercorn async catalog, port 5002 | Active |
+| `nexus-celery.service` | Celery worker (4 prefork) | Active |
+| `nexus-mv-refresh.timer` | Daily MV refresh at 03:00 UTC | Active |
+| `nexus-cache-warm.timer` | Daily Redis cache warming at 04:00 UTC | Active |
+
+## Commands
+
+```bash
+# Reload all
+systemctl daemon-reload
+
+# Restart POS
+systemctl restart nexus-pos.service
+
+# View logs
+journalctl -u nexus-pos.service -f
+```
+
+## Installation
+
+```bash
+sudo cp systemd/*.service systemd/*.timer /etc/systemd/system/
+systemctl daemon-reload
+systemctl enable --now nexus-pos.service nexus-cache-warm.timer
+```