refactor: replace 899 lines of script generation with templates

Created InstallTemplateService with clean template-based script generation.
Added linux.sh.tmpl and windows.ps1.tmpl for install scripts.
Removed massive generateLinuxScript and generateWindowsScript functions.
Downloads handler now uses template service (1073 lines → 174 lines).
Templates easily maintainable without modifying Go code.

aggregator-server/internal/services/templates/install/scripts/linux.sh.tmpl

@@ -0,0 +1,66 @@
+#!/bin/bash
+# RedFlag Agent Installer - Linux
+# Generated for agent: {{.AgentID}}
+# Platform: {{.Platform}}
+# Version: {{.Version}}
+
+set -e
+
+AGENT_ID="{{.AgentID}}"
+BINARY_URL="{{.BinaryURL}}"
+CONFIG_URL="{{.ConfigURL}}"
+INSTALL_DIR="/usr/local/bin"
+CONFIG_DIR="/etc/redflag"
+SERVICE_NAME="redflag-agent"
+VERSION="{{.Version}}"
+
+echo "=== RedFlag Agent v${VERSION} Installation ==="
+echo "Agent ID: ${AGENT_ID}"
+echo "Platform: {{.Platform}}"
+echo "Installing to: ${INSTALL_DIR}/${SERVICE_NAME}"
+echo
+
+# Step 1: Create directories
+echo "Creating directories..."
+sudo mkdir -p "${CONFIG_DIR}"
+sudo mkdir -p "/var/lib/redflag"
+sudo mkdir -p "/var/log/redflag"
+
+# Step 2: Download agent binary
+echo "Downloading agent binary..."
+sudo curl -sSL -o "${INSTALL_DIR}/${SERVICE_NAME}" "${BINARY_URL}"
+sudo chmod +x "${INSTALL_DIR}/${SERVICE_NAME}"
+
+# Step 3: Download configuration
+echo "Downloading configuration..."
+sudo curl -sSL -o "${CONFIG_DIR}/config.json" "${CONFIG_URL}"
+sudo chmod 600 "${CONFIG_DIR}/config.json"
+
+# Step 4: Create systemd service
+echo "Creating systemd service..."
+cat <<EOF | sudo tee /etc/systemd/system/${SERVICE_NAME}.service
+[Unit]
+Description=RedFlag Security Agent
+After=network.target
+
+[Service]
+Type=simple
+User=root
+ExecStart=${INSTALL_DIR}/${SERVICE_NAME}
+Restart=always
+RestartSec=30
+
+[Install]
+WantedBy=multi-user.target
+EOF
+
+# Step 5: Enable and start service
+echo "Enabling and starting service..."
+sudo systemctl daemon-reload
+sudo systemctl enable ${SERVICE_NAME}
+sudo systemctl start ${SERVICE_NAME}
+
+echo
+echo "✓ Installation complete!"
+echo "Agent is running. Check status with: sudo systemctl status ${SERVICE_NAME}"
+echo "View logs with: sudo journalctl -u ${SERVICE_NAME} -f"