feat(manager): add Nexus Instance Manager for demo orchestration

- Complete Flask-based control panel for multi-tenant POS instances
- Dashboard with global stats, system health, and recent demos
- Demo provisioning in 1 click with auto-expiration tracking
- Tenant management: activate/deactivate, reset data, delete
- Health monitoring: PostgreSQL, Redis, disk, memory, systemd services
- Migration orchestration UI for running schema updates across all tenants
- JWT authentication with manager_users table
- Dark theme SPA frontend with real-time search and actions
- systemd service file included

manager/systemd/nexus-manager.service

@@ -0,0 +1,21 @@
+[Unit]
+Description=Nexus Instance Manager (Control Central)
+After=network.target postgresql.service
+
+[Service]
+Type=simple
+User=root
+WorkingDirectory=/home/Autopartes/manager
+ExecStart=/usr/local/bin/gunicorn -w 2 --threads 4 -b 0.0.0.0:5003 "app:create_app()"
+Restart=always
+RestartSec=5
+Environment=PYTHONUNBUFFERED=1
+Environment=PYTHONPATH=/home/Autopartes/manager:/home/Autopartes/pos
+Environment=MASTER_DB_URL=postgresql://postgres@localhost/nexus_autoparts
+Environment=TENANT_DB_URL_TEMPLATE=postgresql://postgres@localhost/{db_name}
+Environment=MANAGER_JWT_SECRET=change-me-to-a-random-64-char-hex-string
+Environment=POS_DIR=/home/Autopartes/pos
+Environment=REDIS_URL=redis://localhost:6379/0
+
+[Install]
+WantedBy=multi-user.target