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.timer

@@ -0,0 +1,9 @@
+[Unit]
+Description=Daily Redis cache warming at 04:00 UTC (1h after MV refresh)
+
+[Timer]
+OnCalendar=*-*-* 04:00:00
+Persistent=true
+
+[Install]
+WantedBy=timers.target