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/install

@@ -0,0 +1,9 @@
+usr/local/bin/pm-web
+usr/local/bin/pm-worker
+usr/local/bin/backup.sh
+usr/share/patch-manager/frontend/*
+usr/share/patch-manager/config.example.toml
+usr/share/patch-manager/migrations/*
+lib/systemd/system/patch-manager-web.service
+lib/systemd/system/patch-manager-worker.service
+lib/systemd/system/patch-manager.target