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.service

@@ -0,0 +1,17 @@
+[Unit]
+Description=Nexus Autoparts Dashboard
+After=network.target postgresql.service
+
+[Service]
+Type=simple
+User=root
+WorkingDirectory=/home/Autopartes/dashboard
+ExecStart=/usr/bin/python3 server.py
+Restart=always
+RestartSec=5
+Environment=PYTHONUNBUFFERED=1
+Environment=DATABASE_URL=postgresql://postgres@localhost/nexus_autoparts
+Environment=JWT_SECRET=nexus-dashboard-jwt-secret-12345678901234567890123456789012
+
+[Install]
+WantedBy=multi-user.target