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/naas.service

@@ -0,0 +1,34 @@
+[Unit]
+Description=No as a Service (NaaS)
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+Restart=always
+RestartSec=10
+TimeoutStopSec=30
+
+# User configuration
+User=naas
+Group=naas
+
+# Load configuration from /etc/default/naas
+EnvironmentFile=-/etc/default/naas
+
+# Binary execution
+ExecStart=/usr/bin/naas
+
+# Security settings
+NoNewPrivileges=true
+PrivateTmp=true
+ProtectSystem=strict
+ProtectHome=true
+ReadWritePaths=
+
+# Resource limits
+MemoryLimit=256M
+CPUQuota=50%
+
+[Install]
+WantedBy=multi-user.target