chore(systemd): remove obsolete systemd service files and update infrastructure documentation

- Remove 8 unused systemd service files from coordinator-api/systemd/
  - aitbc-adaptive-learning.service (port 8005)
  - aitbc-advanced-ai.service
  - aitbc-enterprise-api.service
  - aitbc-gpu-multimodal.service (port 8003)
  - aitbc-marketplace-enhanced.service (port 8006)
  - aitbc-modality-optimization.service (port 8004)
  - aitbc-multimodal.service (port 8002)
  - aitbc-openclaw-enhanced.service (port 8007

systemd/aitbc-enterprise-api.service

@@ -0,0 +1,38 @@
+[Unit]
+Description=AITBC Enterprise API Gateway - Multi-tenant API Management
+After=network.target
+Wants=network.target
+
+[Service]
+Type=simple
+User=aitbc
+Group=aitbc
+WorkingDirectory=/opt/aitbc/apps/coordinator-api
+Environment=PATH=/opt/aitbc/.venv/bin
+Environment=PYTHONPATH=/opt/aitbc/apps/coordinator-api/src
+ExecStart=/opt/aitbc/.venv/bin/python -m app.services.enterprise_api_gateway
+ExecReload=/bin/kill -HUP $MAINPID
+Restart=always
+RestartSec=10
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=aitbc-enterprise-api
+
+# Security settings
+NoNewPrivileges=true
+PrivateTmp=true
+ProtectSystem=strict
+ProtectHome=true
+ReadWritePaths=/opt/aitbc/logs /opt/aitbc/data
+
+# Resource limits
+LimitNOFILE=65536
+LimitNPROC=4096
+
+# Performance settings
+Nice=-5
+IOSchedulingClass=best-effort
+IOSchedulingPriority=0
+
+[Install]
+WantedBy=multi-user.target