Add Debian packaging support with configurable binding

- Add complete debian/ packaging structure for .deb creation
- Support BIND_ADDR environment variable (defaults to 0.0.0.0)
- Create /etc/default/naas config file (defaults to localhost binding)
- Systemd service reads from /etc/default/naas
- Package installs but does not auto-enable service
- Upgrades restart service only if already enabled
- Proper user creation and cleanup on install/remove

Key files:
- debian/control: Package metadata with rustup build dependency support
- debian/naas.default: Configuration template for /etc/default/naas
- debian/naas.service: Updated systemd service with EnvironmentFile
- debian/postinst: Creates naas user, restarts on upgrade if enabled
- debian/prerm: Stops service on removal
- debian/postrm: Cleans up user on purge
- src/main.rs: Added BIND_ADDR support for configurable binding

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

debian/postinst

@@ -0,0 +1,31 @@
+#!/bin/bash
+
+set -e
+
+case "$1" in
+    configure)
+        # Create naas user if it doesn't exist
+        if ! getent passwd naas >/dev/null; then
+            adduser --system --group --no-create-home \
+                --disabled-password --disabled-login \
+                --shell /bin/false naas
+        fi
+
+        # Reload systemd daemon to pick up new service file
+        if [ -d /run/systemd/system ]; then
+            systemctl daemon-reload || true
+        fi
+
+        # On upgrade, restart the service if it's already enabled
+        if [ -n "$2" ] && [ -d /run/systemd/system ]; then
+            if systemctl is-enabled naas >/dev/null 2>&1; then
+                echo "Restarting naas service..."
+                systemctl restart naas || true
+            fi
+        fi
+        ;;
+esac
+
+#DEBHELPER#
+
+exit 0