feat: Add .deb packaging for Ubuntu 24.04 release

- debian/control: Package metadata with dependencies
- debian/postinst: Service user, dirs, JWT key gen, config, cron setup
- debian/prerm: Graceful service stop before upgrade
- debian/postrm: Purge cleanup (user, data, config, cron)
- debian/changelog: 1.0.0-1 initial release
- debian/install: File manifest
- scripts/build-package.sh: Full build pipeline (cargo release, frontend, dpkg-deb)
- .gitignore: Exclude *.deb and package-build/

debian/prerm

@@ -0,0 +1,20 @@
+#!/bin/bash
+set -e
+
+case "$1" in
+    remove|upgrade|deconfigure)
+        # Stop services gracefully
+        if systemctl is-active --quiet patch-manager.target 2>/dev/null; then
+            systemctl stop patch-manager.target 2>/dev/null || true
+        fi
+        ;;
+
+    failed-upgrade)
+        ;;
+
+    *)
+        echo "prerm called with unknown argument \`$1'" >&2
+        ;;
+esac
+
+exit 0