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-cache-warm.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=Warm Redis cache for Nexus vehicle info
+After=postgresql.service redis-server.service
+
+[Service]
+Type=oneshot
+User=postgres
+WorkingDirectory=/home/Autopartes
+ExecStart=/usr/bin/python3 /home/Autopartes/scripts/warm_vehicle_cache.py --batch-size 10000 --ttl 7200
+StandardOutput=append:/var/log/nexus-pos/cache_warm.log
+StandardError=append:/var/log/nexus-pos/cache_warm.log