Update Socket.IO deployment documentation and fixCertPermissions.sh script for improved service user configuration

This commit enhances the Socket.IO deployment documentation by adding a new section on configuring the systemd service to run as `www-data`, ensuring proper permissions for SSL certificate access. It also updates the fixCertPermissions.sh script to handle cases where the service user is not defined or is set to `nobody`, defaulting to `www-data` and verifying its existence. These changes improve the overall security and functionality of the deployment process.

tt-tagebuch.service

@@ -0,0 +1,30 @@
+[Unit]
+Description=TT-Tagebuch Node.js Service
+After=network.target mysql.service
+
+[Service]
+Type=simple
+User=www-data
+Group=www-data
+WorkingDirectory=/var/www/tt-tagebuch.de/backend
+ExecStart=/usr/bin/node server.js
+Restart=always
+RestartSec=10
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=tt-tagebuch
+
+# Umgebungsvariablen
+Environment=NODE_ENV=production
+# PORT: HTTP-Server für API (wird von Apache auf Port 443 proxied)
+Environment=PORT=3050
+# HTTPS_PORT: HTTPS-Server für Socket.IO (direkt erreichbar auf Port 3051)
+Environment=HTTPS_PORT=3051
+
+# Sicherheit
+NoNewPrivileges=true
+PrivateTmp=true
+
+[Install]
+WantedBy=multi-user.target
+