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-pos.service

@@ -0,0 +1,23 @@
+[Unit]
+Description=Nexus POS (Gunicorn)
+After=network.target postgresql.service redis-server.service
+
+[Service]
+Type=simple
+User=root
+WorkingDirectory=/home/Autopartes/pos
+ExecStart=/usr/local/bin/gunicorn -c gunicorn.conf.py "app:create_app()"
+Restart=always
+RestartSec=5
+Environment=PYTHONUNBUFFERED=1
+Environment=PYTHONPATH=/home/Autopartes/pos
+Environment=MASTER_DB_URL=postgresql://postgres@/nexus_autoparts
+Environment=TENANT_DB_URL_TEMPLATE=postgresql://postgres@/nexus_autoparts
+Environment=POS_JWT_SECRET=nexus-pos-jwt-secret-12345678901234567890123456789012
+Environment=REDIS_URL=redis://localhost:6379/0
+Environment=REDIS_ENABLED=true
+Environment=MEILI_URL=http://localhost:7700
+Environment=MEILI_ENABLED=true
+
+[Install]
+WantedBy=multi-user.target