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

systemd/nexus-cache-warm.timer

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+[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

systemd/nexus-celery.service

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+[Unit]
+Description=Nexus POS Celery Worker
+After=network.target postgresql.service redis.service
+
+[Service]
+Type=simple
+User=postgres
+WorkingDirectory=/home/Autopartes/pos
+Environment=MASTER_DB_URL=postgresql://postgres@/nexus_autoparts
+Environment=REDIS_URL=redis://localhost:6379/0
+Environment=PYTHONPATH=/home/Autopartes/pos
+ExecStart=/usr/bin/python3 -m celery -A celery_app worker --loglevel=info --concurrency=4 -n nexus-worker@%h
+Restart=on-failure
+RestartSec=10
+StandardOutput=append:/var/log/nexus-pos/celery.log
+StandardError=append:/var/log/nexus-pos/celery.log
+
+[Install]
+WantedBy=multi-user.target

systemd/nexus-mv-refresh.service

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+[Unit]
+Description=Refresh Nexus part_vehicle_preview materialized view
+After=postgresql.service
+
+[Service]
+Type=oneshot
+User=postgres
+ExecStart=/usr/bin/python3 /home/Autopartes/scripts/refresh_part_vehicle_preview.py
+StandardOutput=append:/var/log/nexus-pos/mv_refresh.log
+StandardError=append:/var/log/nexus-pos/mv_refresh.log

systemd/nexus-mv-refresh.timer

@@ -0,0 +1,9 @@
+[Unit]
+Description=Daily refresh of part_vehicle_preview materialized view at 03:00
+
+[Timer]
+OnCalendar=*-*-* 03:00:00
+Persistent=true
+
+[Install]
+WantedBy=timers.target

systemd/nexus-pos.service

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+[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

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

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