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-quart.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=Nexus Quart Async Catalog (hypercorn)
+After=network.target postgresql.service
+
+[Service]
+Type=simple
+User=root
+WorkingDirectory=/home/Autopartes/pos
+ExecStart=/usr/local/bin/hypercorn async_catalog:app --bind 0.0.0.0:5002
+Restart=always
+RestartSec=5
+Environment=PYTHONUNBUFFERED=1
+Environment=MASTER_DB_URL=postgresql://postgres@localhost/nexus_autoparts
+
+[Install]
+WantedBy=multi-user.target