server: add SMTP config, user management web pages, install script

- SMTP configuration in admin panel with test button
- Admin web interface for user creation (/admin/create-user)
- REST API for user creation (/admin/api/users/create)
- Self-registration endpoint (/register)
- Systemd service file and install script

verstak-server.service

@@ -0,0 +1,22 @@
+[Unit]
+Description=Verstak Sync Server
+After=network.target
+
+[Service]
+Type=simple
+User=verstak
+Group=verstak
+WorkingDirectory=/opt/verstak-sync-server
+ExecStart=/opt/verstak-sync-server/verstak-sync-server --port ${VERSTAK_PORT:-47732} --data /var/lib/verstak-sync-server
+Restart=on-failure
+RestartSec=5
+AmbientCapabilities=CAP_NET_BIND_SERVICE
+NoNewPrivileges=true
+ProtectSystem=full
+ProtectHome=true
+PrivateTmp=true
+StateDirectory=verstak-sync-server
+RuntimeDirectory=verstak-sync-server
+
+[Install]
+WantedBy=multi-user.target