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/

systemd/nexus-celery.service

@@ -0,0 +1,19 @@
+[Unit]
+Description=Nexus POS Celery Worker
+After=network.target postgresql.service redis.service
+
+[Service]
+Type=simple
+User=postgres
+WorkingDirectory=/home/Autopartes/pos
+Environment=MASTER_DB_URL=postgresql://postgres@/nexus_autoparts
+Environment=REDIS_URL=redis://localhost:6379/0
+Environment=PYTHONPATH=/home/Autopartes/pos
+ExecStart=/usr/bin/python3 -m celery -A celery_app worker --loglevel=info --concurrency=4 -n nexus-worker@%h
+Restart=on-failure
+RestartSec=10
+StandardOutput=append:/var/log/nexus-pos/celery.log
+StandardError=append:/var/log/nexus-pos/celery.log
+
+[Install]
+WantedBy=multi-user.target