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.default

@@ -0,0 +1,12 @@
+# Configuration for NaaS (No as a Service)
+# This file is sourced by the systemd service
+
+# Port to listen on (default: 8080)
+PORT=8080
+
+# Bind address (default: 127.0.0.1 for localhost only)
+# Use 0.0.0.0 to listen on all interfaces
+BIND_ADDR=127.0.0.1
+
+# Additional options can be added here as needed
+# RUST_LOG=info